Semiconductor device and operation method thereof

ABSTRACT

A semiconductor device including an amplifier with improved accuracy is provided. The semiconductor device includes a switch, a capacitor, a chopping circuit, and the amplifier. The amplifier includes a non-inverting input terminal, an inverting input terminal, an inverting output terminal, and a non-inverting output terminal. The semiconductor device, with use of the switch and the capacitor, has a function of sampling and holding a first potential and a second potential input in a first period. The chopping circuit is provided on each of the input terminal side and the output terminal side of the amplifier, and the first potential and the second potential are each input to either one of the non-inverting input terminal and the inverting input terminal in a second period. In a third period, the first potential and the second potential are each input to either one of the non-inverting input terminal and the inverted input terminal, which is different from the second period. In a similar manner, the inverting output terminal and non-inverting output terminal are replaced by the chopping circuit in the second period and the third period to be output from the semiconductor device.

TECHNICAL FIELD

The present invention relates to a semiconductor device and an operationmethod thereof. In particular, the present invention relates to asemiconductor device including an amplifier (also referred to as anamplifier circuit) with improved accuracy.

In this specification and the like, a semiconductor device refers to adevice that utilizes semiconductor characteristics, such as a circuitincluding a semiconductor element (a transistor, a diode, a photodiode,and the like), a device including the circuit, and the like. In thisspecification and the like, a semiconductor device refers to any devicethat can function by utilizing semiconductor characteristics; examplesof the semiconductor device include an integrated circuit, a chipprovided with an integrated circuit, an electronic component in which achip is incorporated in a package, and an electronic device providedwith an integrated circuit.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of the invention disclosed inthis specification and the like relates to an object, a method, or amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition of matter.

BACKGROUND ART

As one of techniques of a semiconductor circuit, a technique of aswitched capacitor circuit has been known in which a switch (alsoreferred to as a switching element) is combined with a capacitor (alsoreferred to as a capacitor element) and the switch controls charge anddischarge of the capacitor. Temperature dependence of the electriccharacteristics of a switched capacitor circuit is small, and theswitched capacitor circuit can be used in place of a resistor (alsoreferred to as a resistor element) in a semiconductor circuit; thus, asemiconductor device having small temperature dependence can beachieved.

In addition, a technique of using a switched capacitor circuit and anamplifier in combination has been known (see Non-Patent Document 1). Asemiconductor device in which a switched capacitor circuit and anamplifier are combined (such a semiconductor device is also referred toas a switched capacitor amplifier) can, with a signal (potential) to beinput to the semiconductor device being sampled and held in a capacitor,achieve a highly accurate amplifier.

Meanwhile, a transistor including an oxide semiconductor or a metaloxide in a channel formation region (also referred to as an oxidesemiconductor (OS) transistor) has been attracting attention. The draincurrent of an OS transistor in an off state (such a current is alsoreferred to as an off-state current) is extremely low (e.g., seeNon-Patent Documents 2 and 3); when an OS transistor is used in a memorycell of a DRAM, for example, electric charge accumulated in a capacitiveelement can be retained for a long time.

A CAAC (c-axis aligned crystalline) structure and an nc(nanocrystalline) structure, which are neither single crystal noramorphous, have been found in an oxide semiconductor (see Non-PatentDocument 2 and Non-Patent Document 4). Non-Patent Document 2 andNon-Patent Document 4 also disclose a technique for fabricating atransistor using an oxide semiconductor having a CAAC structure.

REFERENCE Non-Patent Document

-   [Non-Patent Document 1] “Design of Analog CMOS Integrated Circuits    (Application)”, Behzad Razavi (supervised and translated by Tadahiro    Kuroda), Maruzen Publishing Co., Ltd. (March 2003), pp. 495-498.-   [Non-Patent Document 2] S. Yamazaki et al., “Properties of    crystalline In-Ga-Zn-oxide semiconductor and its transistor    characteristics”, Jpn. J. Appl. Phys., vol. 53, 04ED18 (2014).-   [Non-Patent Document 3] K. Kato et al., “Evaluation of Off-State    Current Characteristics of Transistor Using Oxide Semiconductor    Material, Indium-Gallium-Zinc Oxide”, Jpn. J. Appl. Phys., vol. 51,    021201 (2012).-   [Non-Patent Document 4] S. Yamazaki et al., “SID Symposium Digest of    Technical Papers”, 2012, volume 43, issue 1, pp. 183-186

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In an amplifier including a non-inverting input terminal and aninverting input terminal, for example, amplifier-induced noise such asoffset voltage which is output even when a potential difference betweenthe non-inverting input terminal and the inverting input terminal is 0V, 1/f noise for which the noise power is inversely proportional to thefrequency and which can hardly be filtered out, or thermal noise whichis caused when a free electron moves randomly with thermal energy issuperimposed on the output even in a period when a switched capacitoramplifier is sampling an input signal and holding the signal in thecapacitor, and the removal of such noise is difficult, which has beenproblematic.

An object of one embodiment of the present invention is to provide aswitched capacitor amplifier in which an effect of amplifier-inducednoise on an output is reduced. Another object of one embodiment of thepresent invention is to provide a semiconductor device including anamplifier, in which an effect of amplifier-induced noise on an output isreduced. Another object of one embodiment of the present invention is toprovide a semiconductor device including an amplifier, in which theaccuracy of the amplifier is improved.

Note that one embodiment of the present invention does not necessarilyhave to achieve all the above-described objects and only needs toachieve at least one of the objects. The descriptions of the aboveobjects do not preclude the existence of other objects. Objects otherthan these will be apparent from the descriptions of the specification,the claims, the drawings, and the like, and objects other than these canbe derived from the descriptions of the specification, the claims, thedrawings, and the like.

Means for Solving the Problems

One embodiment of the present invention is a semiconductor deviceincluding a switch, first and second capacitors, first and secondchopping circuits, an amplifier, first and second input terminals, andfirst and second output terminals. The amplifier includes anon-inverting input terminal, an inverting input terminal, an invertingoutput terminal, and a non-inverting output terminal. In a first period,the semiconductor device electrically connects the first input terminaland one terminal of the first capacitor, electrically connects thesecond input terminal and one terminal of the second capacitor,electrically connects the other terminal of the first capacitor and thefirst output terminal, and electrically connects the other terminal ofthe second capacitor and the second output terminal; the first choppingcircuit electrically connects the other terminal of the first capacitorand the non-inverting input terminal and electrically connects the otherterminal of the second capacitor and the inverting input terminal; andthe second chopping circuit electrically connects the inverting outputterminal and the first output terminal and electrically connects thenon-inverting output terminal and the second output terminal. In asecond period, the semiconductor device electrically connects the oneterminal of the first capacitor and the first output terminal andelectrically connects the one terminal of the second capacitor and thesecond output terminal; the first chopping circuit electrically connectsthe other terminal of the first capacitor and the non-inverting inputterminal and electrically connects the other terminal of the secondcapacitor and the inverting input terminal, and the second choppingcircuit electrically connects the inverting output terminal and thefirst output terminal and electrically connects the non-inverting outputterminal and the second output terminal. In a third period, thesemiconductor device electrically connects the one terminal of the firstcapacitor and the first output terminal and electrically connects theone terminal of the second capacitor and the second output terminal; thefirst chopping circuit electrically connects the other terminal of thefirst capacitor and the inverting input terminal and electricallyconnects the other terminal of the second capacitor and thenon-inverting input terminal; and the second chopping circuitelectrically connects the non-inverting output terminal and the firstoutput terminal and electrically connects the inverting output terminaland the second output terminal.

In the above embodiment, the switch, the first chopping circuit, and thesecond chopping circuit each include a transistor, and the transistorincludes a metal oxide in a channel formation region.

Another embodiment of the present invention is an operation method of asemiconductor device including a switch, first and second capacitors,first and second chopping circuits, an amplifier, first and second inputterminals, and first and second output terminals. The amplifier includesa non-inverting input terminal, an inverting input terminal, aninverting output terminal, and a non-inverting output terminal. In afirst period, the semiconductor device electrically connects the firstinput terminal and one terminal of the first capacitor, electricallyconnects the second input terminal and one terminal of the secondcapacitor, electrically connects the other terminal of the firstcapacitor and the first output terminal, and electrically connects theother terminal of the second capacitor and the second output terminal;the first chopping circuit electrically connects the other terminal ofthe first capacitor and the non-inverting input terminal andelectrically connects the other terminal of the second capacitor and theinverting input terminal; and the second chopping circuit electricallyconnects the inverting output terminal and the first output terminal andelectrically connects the non-inverting output terminal and the secondoutput terminal. In a second period, the semiconductor deviceelectrically connects the one terminal of the first capacitor and thefirst output terminal and electrically connects the one terminal of thesecond capacitor and the second output terminal; the first choppingcircuit electrically connects the other terminal of the first capacitorand the non-inverting input terminal and electrically connects the otherterminal of the second capacitor and the inverting input terminal, andthe second chopping circuit electrically connects the inverting outputterminal and the first output terminal and electrically connects thenon-inverting output terminal and the second output terminal. In a thirdperiod, the semiconductor device electrically connects the one terminalof the first capacitor and the first output terminal and electricallyconnects the one terminal of the second capacitor and the second outputterminal; the first chopping circuit electrically connects the otherterminal of the first capacitor and the inverting input terminal andelectrically connects the other terminal of the second capacitor and thenon-inverting input terminal; and the second chopping circuitelectrically connects the non-inverting output terminal and the firstoutput terminal and electrically connects the inverting output terminaland the second output terminal.

In the above embodiment, the switch, the first chopping circuit, and thesecond chopping circuit each include a transistor, and the transistorincludes a metal oxide in a channel formation region.

Another embodiment of the present invention is a semiconductor deviceincluding first to sixth switches, first and second capacitors, firstand second chopping circuits, an amplifier, first and second inputterminals, and first and second output terminals. The amplifier includesa non-inverting input terminal, an inverting input terminal, an invertedoutput terminal, and a non-inverting output terminal; the first choppingcircuit includes first to fourth terminals; and the second choppingcircuit includes fifth to eighth terminals. The first input terminal iselectrically connected to one terminal of the first switch, the secondinput terminal is electrically connected to one terminal of the secondswitch, the other terminal of the first switch is electrically connectedto one terminal of the third switch and one terminal of the firstcapacitor, the other terminal of the second switch is electricallyconnected to one terminal of the fourth switch and one terminal of thesecond capacitor, the other terminal of the first capacitor iselectrically connected to one terminal of the fifth switch and the firstterminal, and the other terminal of the second capacitor is electricallyconnected to one terminal of the sixth switch and the second terminal.The third terminal is electrically connected to the non-inverting inputterminal, the fourth terminal is electrically connected to the invertinginput terminal, the inverting output terminal is electrically connectedto the fifth terminal, the non-inverting output terminal is electricallyconnected to the sixth terminal, the seventh terminal is electricallyconnected to the other terminal of the third switch, the other terminalof the fifth switch, and the first output terminal, and the eighthterminal is electrically connected to the other terminal of the fourthswitch, the other terminal of the sixth switch, and the second outputterminal. In a first period, the first chopping circuit has a functionof bringing the first terminal and the third terminal into a conductionstate and a function of bringing the second terminal and the fourthterminal into a conduction state, and the second chopping circuit has afunction of bringing the fifth terminal and the seventh terminal into aconduction state and a function of bringing the sixth terminal and theeighth terminal into a conduction state. In a second period, the firstchopping circuit has a function of bringing the first terminal and thefourth terminal into a conduction state and a function of bringing thesecond terminal and the third terminal into a conduction state, and thesecond chopping circuit has a function of bringing the fifth terminaland the eighth terminal into a conduction state and a function ofbringing the sixth terminal and the seventh terminal into a conductionstate.

In the above embodiment, the first to sixth switches, the first choppingcircuit, and the second chopping circuit each include a transistor, andthe transistor includes a metal oxide in a channel formation region.

Effect of the Invention

According to one embodiment of the present invention, a switchedcapacitor amplifier in which an effect of amplifier-induced noise on anoutput is reduced can be provided. Alternatively, according to oneembodiment of the present invention, a semiconductor device including anamplifier, in which an effect of amplifier-induced noise on an output isreduced, can be provided. Alternatively, according to one embodiment ofthe present invention, a semiconductor device including an amplifier, inwhich the accuracy of the amplifier is improved, can be provided.

Note that the descriptions of the above effects do not preclude theexistence of other effects. Note that one embodiment of the presentinvention does not necessarily have to achieve all the above effects andonly needs to have at least one of the effects. Effects other than theseare apparent from the descriptions of the specification, the claims, thedrawings, and the like, and effects other than these can be derived fromthe descriptions of the specification, the claims, the drawings, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing a configuration example of asemiconductor device. FIG. 1B, FIG. 1D, and FIG. 1F are diagrams eachshowing a symbol representing a switch. FIG. 1C, FIG. 1E, and FIG. 1Gare circuit diagrams each showing a configuration example of the switch.

FIG. 2A is a diagram showing a symbol representing a chopping circuit.FIG. 2B is a circuit diagram showing a configuration example of thechopping circuit. FIG. 2C is a diagram showing a symbol representing anamplifier. FIG. 2D is a circuit diagram showing a configuration exampleof the amplifier.

FIG. 3 is a timing chart showing an operation example of a semiconductordevice.

FIG. 4A to FIG. 4C are diagrams each showing an equivalent circuit of asemiconductor device.

FIG. 5 is a cross-sectional view showing a structure example of asemiconductor device.

FIG. 6A to FIG. 6C are cross-sectional views showing a structure exampleof a transistor.

FIG. 7A is a top view showing a structure example of a transistor. FIG.7B and FIG. 7C are cross-sectional views showing a structure example ofthe transistor.

FIG. 8A is a top view showing a structure example of a transistor. FIG.8B and FIG. 8C are cross-sectional views showing a structure example ofthe transistor.

FIG. 9A is a top view showing a structure example of a transistor. FIG.9B and FIG. 9C are cross-sectional views showing a structure example ofthe transistor.

FIG. 10A is a top view showing a structure example of a transistor. FIG.10B and FIG. 10C are cross-sectional views showing a structure exampleof the transistor.

FIG. 11A is a top view showing a structure example of a transistor. FIG.11B and FIG. 11C are cross-sectional views showing a structure exampleof the transistor.

FIG. 12A is a top view showing a structure example of a transistor. FIG.12B and FIG. 12C are cross-sectional views showing a structure exampleof the transistor.

FIG. 13A and FIG. 13B are cross-sectional views showing a structureexample of a transistor.

FIG. 14 is a cross-sectional view showing a structure example of asemiconductor device.

FIG. 15A and FIG. 15B are cross-sectional views showing a structureexample of a transistor.

FIG. 16A is a diagram describing the classification of crystalstructures of IGZO. FIG. 16B is a graph showing an XRD spectrum of aCAAC-IGZO film. FIG. 16C is an image showing a nanobeam electrondiffraction pattern of a CAAC-IGZO film.

MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the drawings.However, the embodiments can be implemented in many different modes, andit will be readily appreciated by those skilled in the art that modesand details thereof can be changed in various ways without departingfrom the spirit and scope thereof. Thus, the present invention shouldnot be construed as being limited to the following description of theembodiments.

A plurality of embodiments described below can be combined asappropriate. In addition, in the case where a plurality of structureexamples are described in one embodiment, the structure examples can becombined as appropriate.

Note that in the drawings attached to this specification, the blockdiagram in which components are classified according to their functionsand shown as independent blocks is illustrated; however, it is difficultto completely separate actual components according to their functions,and it is possible for one component to relate to a plurality offunctions.

In the drawings and the like, the size, the layer thickness, the region,or the like is exaggerated for clarity in some cases. Therefore, theyare not limited to the illustrated scale. The drawings schematicallyshow ideal examples, and shapes, values, or the like are not limited toshapes, values, or the like shown in the drawings.

In the drawings and the like, the same elements, elements having similarfunctions, elements formed of the same material, elements formed at thesame time, or the like are sometimes denoted by the same referencenumerals, and description thereof is not repeated in some cases.

In this specification and the like, the term “film” and the term “layer”can be interchanged with each other. For example, the term “conductivelayer” can be changed into the term “conductive film” in some cases. Asanother example, the term “insulating film” can be changed into the term“insulating layer” in some cases.

In this specification and the like, the terms for describing arrangementsuch as “over” and “below” do not necessarily mean “directly over” and“directly below”, respectively, in the positional relationship betweencomponents. For example, the expression “a gate electrode over a gateinsulating layer” does not exclude the case where there is an additionalcomponent between the gate insulating layer and the gate electrode.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification and the like, when a plurality of components aredenoted by the same reference signs, and in particular need to bedistinguished from each other, an identification sign such as “_1”,“_2”,“[n]”, or “[m, n]” is sometimes added to the reference signs. Forexample, the second wiring GL is referred to as a wiring GL[2].

In this specification and the like, “electrically connected” includesthe case where connection is made through an “object having any electricfunction”. There is no particular limitation on the “object having anyelectric function” as long as electric signals can be transmitted andreceived between components that are connected through the object.Examples of the “object having any electric function” include aswitching element such as a transistor, a resistor, an inductor, acapacitive element, and other elements with a variety of functions aswell as an electrode and a wiring. Furthermore, even when the expression“being electrically connected” is used, there is a case in which nophysical connection portion is made and a wiring is just extended in anactual circuit.

In addition, in this specification and the like, the term “electrode” or“wiring” does not functionally limit these components. For example, an“electrode” is used as part of a “wiring” in some cases, and vice versa.

In this specification and the like, a “terminal” in an electric circuitrefers to a portion where a current or a potential is input (or output)or a signal is received (or transmitted). Accordingly, part of a wiringor an electrode functions as a terminal in some cases.

In general, a “capacitive element” has a structure in which twoelectrodes face each other with an insulator (dielectric) therebetween.Furthermore, in this specification and the like, cases where a“capacitive element” is one having a structure in which two electrodesface each other with an insulator therebetween, one having a structurein which two wirings face each other with an insulator therebetween, orone in which two wirings are positioned with an insulator therebetween,are included. In this specification and the like, a “capacitive element”is referred to as a “capacitor” or a “condenser” in some cases.

In this specification and the like, a “voltage” often refers to apotential difference between a given potential and a reference potential(e.g., a ground potential). Thus, a voltage and a potential differencecan be interchanged with each other.

In this specification and the like, a transistor is an element having atleast three terminals of a source, a drain, and a gate. Further, achannel formation region is included between the source (a sourceterminal, a source region, or a source electrode) and the drain (a drainterminal, a drain region, or a drain electrode), and a current can flowbetween the source and the drain through the channel formation region.Note that in this specification and the like, a channel formation regionrefers to a region through which a current mainly flows.

Furthermore, functions of a source and a drain might be switched when atransistor of opposite polarity is employed or the direction of currentflow is changed in circuit operation, for example. Thus, the terms of asource and a drain are interchangeable in this specification and thelike.

Furthermore, unless otherwise specified, an off-state current in thisspecification and the like refers to a drain current of a transistor inan off state (also referred to as a non-conduction state or a cutoffstate). Unless otherwise specified, the off state of an n-channeltransistor refers to a state where the voltage Vgs of a gate withrespect to a source is lower than a threshold voltage Vth, and the offstate of a p-channel transistor refers to a state where the voltage Vgsof a gate with respect to a source is higher than the threshold voltageVth. That is, the off-state current of an n-channel transistor sometimesrefers to a drain current at the time when the voltage Vgs of a gatewith respect to a source is lower than the threshold voltage Vth.

In the above description of the off-state current, the drain may bereplaced with the source. That is, the off-state current sometimesrefers to a source current when a transistor is in the off state. Inaddition, a leakage current sometimes expresses the same meaning as theoff-state current. Furthermore, in this specification and the like, theoff-state current sometimes refers to a current that flows between asource and a drain when a transistor is in the off state.

Furthermore, in this specification and the like, an on-state currentsometimes refers to a current that flows between a source and a drainwhen a transistor is in the on state (also referred to as a conductionstate).

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor, and the like.

For example, in the case where a metal oxide is used in a channelformation region of a transistor, the metal oxide is referred to as anoxide semiconductor in some cases. That is to say, in the case where ametal oxide has at least one of an amplifying function, a rectifyingfunction, and a switching function, the metal oxide can be referred toas a metal oxide semiconductor. In other words, a transistor including ametal oxide in a channel formation region can be referred to as an“oxide semiconductor transistor” or an “OS transistor”. Similarly, a“transistor using an oxide semiconductor” is also a transistor includinga metal oxide in a channel formation region.

Furthermore, in this specification and the like, a metal oxidecontaining nitrogen is also referred to as a metal oxide in some cases.A metal oxide containing nitrogen may be referred to as a metaloxynitride. The details of a metal oxide will be described later.

Embodiment 1

In this embodiment, a configuration example and an operation example ofa semiconductor device related to one embodiment of the presentinvention will be described.

<Configuration Example of Semiconductor Device>

FIG. 1A is a block diagram showing a configuration example of asemiconductor device 100 according to one embodiment of the presentinvention. The semiconductor device 100 includes a switch SW1_1, aswitch SW1_2, a switch SW2_1, a switch SW2_2, a switch SW3_1, a switchSW3_2, a capacitor C11, a capacitor C12, a chopping circuit 20_1, achopping circuit 20_2, and an amplifier 30.

The semiconductor device 100 also includes an input terminal INP, aninput terminal INM, an output terminal OUTP, and an output terminalOUTM. The chopping circuit 20_1, the chopping circuit 20_2, and theamplifier 30 each include a first terminal to a fourth terminal. Thefirst terminal to the fourth terminal included in each of the choppingcircuit 20_1, the chopping circuit 20_2, and the amplifier 30 will bedescribed later.

Note that in this specification and the like, a reference numeral suchas “_1” or “_2” is used to distinguish a plurality of components havingsimilar functions. That is, the reference character “SW1” is used torefer to either one of the switch SW1_1 and the switch SW1_2 withoutspecifying which, and the reference character “SW1_1” or “SW1_2” is usedto refer to a specific one of the switch SW1_1 and the switch SW1_2.

In this specification and the like, expressions such as “inputterminal”, “output terminal”, and “terminal” are used in order todescribe input and output of signals and potentials between components;however, in some cases physical connecting portions such as “inputterminal”, “output terminal”, and “terminal” do not exist in the actualcircuit and the components are just electrically connected to each othervia wirings, electrodes, or the like.

In the semiconductor device 100, the input terminal INP is electricallyconnected to one terminal of the switch SW1_1, the input terminal INM iselectrically connected to one terminal of the switch SW1_2, the otherterminal of the switch SW1_1 is electrically connected to one terminalof the switch SW3_1 and one terminal of the capacitor C11, and the otherterminal of the switch SW1_2 is electrically connected to one terminalof the switch SW3_2 and one terminal of the capacitor C12.

The other terminal of the capacitor C11 is electrically connected to oneterminal of the switch SW2_1 and the first terminal of the choppingcircuit 20_1, the other terminal of the capacitor C12 is electricallyconnected to one terminal of the switch SW2_2 and the second terminal ofthe chopping circuit 20_1, the third terminal of the chopping circuit20_1 is electrically connected to the first terminal of the amplifier30, and the fourth terminal of the chopping circuit 20_1 is electricallyconnected to the second terminal of the amplifier 30.

The third terminal of the amplifier 30 is electrically connected to thefirst terminal of the chopping circuit 20_2; the fourth terminal of theamplifier 30 is electrically connected to the second terminal of thechopping circuit 20_2; the third terminal of the chopping circuit 20_2is electrically connected to the other terminal of the switch SW2_1, theother terminal of the switch SW3_1, and the output terminal OUTM; andthe fourth terminal of the chopping circuit 20_2 is electricallyconnected to the other terminal of the switch SW2_2, the other terminalof the switch SW3_2, and the output terminal OUTP.

<Configuration Example of Switch>

The switch SW1 can be formed using a transistor 11, for example. FIG. 1Bis a diagram showing a symbol representing the switch SW1, and FIG. 1Cis a circuit diagram showing a configuration example of the switch SW1.Note that in FIG. 1B and the like, two terminals of the switch SW1 arereferred to as a terminal T11 and a terminal T12.

As shown in FIG. 1C, the switch SW1 includes the transistor 11, one of asource and a drain of the transistor 11 is electrically connected to theterminal T11, and the other of the source and the drain of thetransistor 11 is electrically connected to the terminal T12. A signal S1is input to a gate of the transistor 11, and the switch SW1 is a switchwhose conduction state or non-conduction state is controlled by thesignal S10. That is, when the signal S1 is at a high level, the terminalT11 and the terminal T12 are brought into a conduction state, and whenthe signal S1 is at a low level, the terminal T11 and the terminal T12are brought into a non-conduction state.

The switch SW2 can be formed using a transistor 12, for example. FIG. 1Dis a diagram showing a symbol representing the switch SW2, and FIG. 1Eis a circuit diagram showing a configuration example of the switch SW2.Note that in FIG. 1D and the like, two terminals of the switch SW2 arereferred to as a terminal T13 and a terminal T14. A configurationexample of the switch SW2 is similar to that of the switch SW1, so thedescription thereof is omitted. The switch SW2 is a switch whoseconduction state or non-conduction state is controlled by a signal S2;when the signal S2 is at the high level, the terminal T13 and theterminal T14 are brought into a conduction state, and when the signal S2is at the low level, the terminal T13 and the terminal T14 are broughtinto a non-conduction state.

The switch SW3 can be formed using a transistor 13, for example. FIG. 1Fis a diagram showing a symbol representing the switch SW3, and FIG. 1Gis a circuit diagram showing a configuration example of the switch SW3.Note that in FIG. 1F and the like, two terminals of the switch SW3 arereferred to as a terminal T15 and a terminal T16. A configurationexample of the switch SW3 is similar to that of the switch SW1, so thedescription thereof is omitted. The switch SW3 is a switch whoseconduction state or non-conduction state is controlled by a signal S3;when the signal S3 is at the high level, the terminal T15 and theterminal T16 are brought into a conduction state, and when the signal S3is at the low level, the terminal T15 and the terminal T16 are broughtinto a non-conduction state.

<Configuration Example of Chopping Circuit>

The chopping circuit 20 can be formed using a transistor 21 to atransistor 24, for example. FIG. 2A is a diagram showing a symbolrepresenting the chopping circuit 20, and FIG. 2B is a circuit diagramshowing a configuration example of the chopping circuit 20.

In FIG. 2A and the like, four terminals of the chopping circuit 20 arereferred to as a terminal T21 to a terminal T24. In the chopping circuit20, the terminal T21 corresponds to the above-mentioned first terminal,the terminal T22 corresponds to the above-mentioned second terminal, theterminal T23 corresponds to the above-mentioned third terminal, and theterminal T24 corresponds to the above-mentioned fourth terminal.

As shown in FIG. 2B, the chopping circuit 20 includes the transistor 21to the transistor 24, the terminal T21 is electrically connected to oneof a source and a drain of the transistor 21 and one of a source and adrain of the transistor 22, and the terminal T22 is electricallyconnected to one of a source and a drain of the transistor 23 and one ofa source and a drain of the transistor 24. The terminal T23 iselectrically connected to the other of the source and the drain of thetransistor 21 and the other of the source and the drain of thetransistor 24, and the terminal T24 is electrically connected to theother of the source and the drain of the transistor 23 and the other ofthe source and the drain of the transistor 22.

A signal S4 is input to a gate of the transistor 21 and a gate of thetransistor 23, and a conduction state or a non-conduction state of eachof the transistor 21 and the transistor 23 is controlled by the signalS4. A signal S5 is input to a gate of the transistor 22 and a gate ofthe transistor 24, and a conduction state or a non-conduction state ofeach of the transistor 22 and the transistor 24 is controlled by thesignal S5.

The signal S5 is an inverted signal of the signal S4. When the signal S4is at the high level, the signal S5 is at the low level; and when thesignal S4 is at the low level, the signal S5 is at the high level. Thatis, when the signal S4 is at the high level, the terminal T21 and theterminal T23 are in a conduction state, and the terminal T22 and theterminal T24 are in a conduction state. When the signal S4 is at the lowlevel, the terminal T21 and the terminal T24 are in a conduction state,and the terminal T22 and the terminal T23 are in a conduction state.

<Transistor 1>

OS transistors can be used as the transistor 11 to the transistor 13,and the transistor 21 to the transistor 24. An oxide semiconductor has aband gap of 2 eV or more and thus has a characteristic of an off-statecurrent being extremely small. In an OS transistor, for example, anormalized off-state current per micrometer of channel width at asource-drain voltage of 10 V can be less than or equal to 10×10⁻²¹ A (10zeptoampere). Note that the details of the OS transistor will bedescribed in Embodiment 2 and Embodiment 3.

An OS transistor has the following features. An OS transistor can beformed by a method such as a thin-film method, and thus can be providedover a semiconductor substrate. An off-state current of an OS transistordoes not easily increase even under high temperature environments, sothat the switch SW1 to the switch SW3 can be highly reliable switches,for example. OS transistors can be manufactured by using fabricationtools that are similar to those of transistors with silicon in a channelformation region, which enables OS transistors to be manufactured at lowcost.

The transistor 11 to the transistor 13, and the transistor 21 to thetransistor 24 may each have a back gate (also referred to as a secondgate or a bottom gate). In the case where the transistor 11 has a backgate, for example, the threshold voltage of the transistor 11 can beincreased and decreased by application of a predetermined potential tothe back gate of the transistor 11. Alternatively, when the back gate ofthe transistor 11 is electrically connected to the gate (also referredto as a first gate, a top gate, or a front gate with respect to the backgate) of the transistor 11, the on-state current of the transistor 11can be increased.

A metal oxide used in the channel formation region of an OS transistoris preferably an oxide containing one or more elements selected from In,Ga, Sn, and Zn. As such an oxide, an In—Sn—Ga—Zn oxide, an In—Ga—Znoxide, an In—Sn—Zn oxide, an In—Al—Zn oxide, a Sn—Ga—Zn oxide, anAl—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Zn oxide, a Sn—Zn oxide, anAl—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Gaoxide, an In oxide, a Sn oxide, a Zn oxide, or the like can be used.

Alternatively, as each of the transistor 11 to the transistor 13 and thetransistor 21 to the transistor 24, a transistor other than an OStransistor may be used. The transistor 11 to the transistor 13 and thetransistor 21 to the transistor 24 are each preferably a transistor withsmall off-state current, and for example, a transistor in which asemiconductor with a wide bandgap is included in a channel formationregion can be used. The semiconductor with a wide bandgap refers to asemiconductor whose bandgap is larger than or equal to 2.2 eV in somecases, and examples thereof include silicon carbide, gallium nitride,and diamond.

<Configuration Example of Amplifier>

The amplifier 30 can be formed using a transistor 31 to a transistor 39and a transistor 41 to a transistor 44, for example. FIG. 2C is adiagram showing a symbol representing the amplifier 30, and FIG. 2D is acircuit diagram showing a configuration example of the amplifier 30.

In FIG. 2C and the like, four terminals of the amplifier 30 are referredto as a terminal T31 to a terminal T34. In the amplifier 30, theterminal T31 corresponds to the above-mentioned first terminal, theterminal T32 corresponds to the above-mentioned second terminal, theterminal T33 corresponds to the above-mentioned third terminal, and theterminal T34 corresponds to the above-mentioned fourth terminal. Theterminal T31 can have a property of a non-inverting input terminal ofthe amplifier 30, the terminal T32 can have a property of an invertinginput terminal of the amplifier 30, the terminal T33 can have a propertyof an inverting output terminal of the amplifier 30, and the terminalT34 can have a property of a non-inverting output terminal of theamplifier 30.

As shown in FIG. 2D, the amplifier 30 includes a terminal T_VDD, aterminal T_BP, a terminal T_CP, a terminal T_COM, a terminal T_CN, and aterminal T_BN.

As shown in FIG. 2D, the amplifier 30 includes the transistor 31 to thetransistor 39 and the transistor 41 to the transistor 44; one of asource and a drain of the transistor 31 is electrically connected to theterminal T_VDD; and the other of the source and the drain of thetransistor 31 is electrically connected to one of a source and a drainof the transistor 32, one of a source and a drain of the transistor 33,one of a source and a drain of the transistor 34, and one of a sourceand a drain of the transistor 35.

The other of the source and the drain of the transistor 32 iselectrically connected to one of a source and a drain of the transistor41 and one of a source and a drain of the transistor 43, and the otherof the source and the drain of the transistor 33 is electricallyconnected to a wiring supplied with a reference potential. The other ofthe source and the drain of the transistor 34 is electrically connectedto the wiring supplied with the reference potential, and the other ofthe source and the drain of the transistor 35 is electrically connectedto one of a source and a drain of the transistor 42 and one of a sourceand a drain of the transistor 44.

One of a source and a drain of the transistor 36 and one of a source anda drain of the transistor 37 are electrically connected to the terminalT_VDD, the other of the source and the drain of the transistor 36 iselectrically connected to one of a source and a drain of the transistor38, and the other of the source and the drain of the transistor 37 iselectrically connected to one of a source and a drain of the transistor39. The other of the source and the drain of the transistor 38 iselectrically connected to the terminal T33 and the other of the sourceand the drain of the transistor 41, the other of the source and thedrain of the transistor 39 is electrically connected to the terminal T34and the other of the source and the drain of the transistor 42, and theother of the source and the drain of the transistor 43 and the other ofthe source and the drain of the transistor 44 are electrically connectedto the wiring supplied with the reference potential.

A gate of the transistor 31, a gate of the transistor 36, and a gate ofthe transistor 37 are electrically connected to the terminal T_BP; agate of the transistor 38 and a gate of the transistor 39 areelectrically connected to the terminal T_CP; a gate of the transistor 32is electrically connected to the terminal T31; and a gate of thetransistor 35 is electrically connected to the terminal T32. A gate ofthe transistor 33 and a gate of the transistor 34 are electricallyconnected to the terminal T_COM, a gate of the transistor 41 and a gateof the transistor 42 are electrically connected to the terminal T_CN,and a gate of the transistor 43 and a gate of the transistor 44 areelectrically connected to the terminal T_BN.

A power supply potential VDD is input to the terminal T_VDD, and a biaspotential that adjusts the operation of the amplifier 30 is input toeach of the terminal T_BP, the terminal T_CP, the terminal T_COM, theterminal T_CN, and the terminal T_BN. A potential at the middle ofpotentials input to the terminal T31 and the terminal T32 is preferablyinput to the terminal T_COM, for example.

<Transistor 2>

Transistors formed on a semiconductor substrate can be used as thetransistor 31 to the transistor 39 and the transistor 41 to thetransistor 44. There is no particular limitation on the semiconductorsubstrate as long as a channel region of a transistor can be formedtherein. For example, a single crystal silicon substrate, a singlecrystal germanium substrate, a compound semiconductor substrate (such asa SiC substrate or a GaN substrate), an SOI (Silicon on Insulator)substrate, or the like can be used.

As the SOI substrate, the following substrate may be used for example:an SIMOX (Separation by Implanted Oxygen) substrate which is formed insuch a manner that after an oxygen ion is implanted into amirror-polished wafer, an oxide layer is formed at a certain depth fromthe surface and defects generated in a surface layer are eliminated byhigh-temperature annealing; or an SOI substrate formed by using a methodsuch as an ELTRAN method (a registered trademark: Epitaxial LayerTransfer) or a Smart-Cut method in which a semiconductor substrate iscleaved by utilizing growth of a minute void, which is formed byimplantation of a hydrogen ion, by thermal treatment. A transistorformed using a single crystal substrate includes a single crystalsemiconductor in a channel formation region.

In this embodiment, an example in which a single crystal siliconsubstrate is used as the semiconductor substrate will be described. Atransistor formed on a single crystal silicon substrate is referred toas a “Si transistor”. In the configuration example of the amplifier 30shown in FIG. 2D, the transistor 31 to the transistor 39 are p-channeltransistors, and the transistor 41 to the transistor 44 are n-channeltransistors.

<Operation Example of Semiconductor Device>

FIG. 3 is a timing chart showing an operation example of thesemiconductor device 100. The timing chart in FIG. 3 shows potentialstates (the high level or the low level) of the signal S1 to the signalS5 from Time T1 to Time T10.

At Time T1, the signal S1 and the signal S2 change from the low level tothe high level. The signal S3 and the signal S5 stay at the low level,and the signal S4 stays at the high level. That is, the switch SW1 andthe switch SW2 are in a conduction state, and the switch SW3 is in anon-conduction state. In a period from Time T1 to Time T2, sampling ofthe potentials input to the input terminal INP and the input terminalINM is performed.

With the sampling being performed, when a charge +Q11 is stored in theone terminal of the capacitor C11, a charge −Q11 is stored in the otherterminal of the capacitor C11. In a similar manner, when a charge +Q12is stored in the one terminal of the capacitor C12, a charge −Q12 isstored in the other terminal of the capacitor C12.

In a period from Time T1 to Time T5, the first terminal and the thirdterminal of each of the chopping circuit 20_1 and the chopping circuit20_2 are in a conduction state, and the second terminal and the fourthterminal of each of the chopping circuit 20_1 and the chopping circuit20_2 are in a conduction state. FIG. 4A shows an equivalent circuit ofthe semiconductor device 100 in which the states of the switch SW1 tothe switch SW3, the chopping circuit 20_1, and the chopping circuit 20_2in the period from Time T1 to Time T2 are reflected.

In this state, the signal S2 changes from the high level to the lowlevel at Time T2, thereby turning the switch SW2 into a non-conductionstate. The signal S1 changes from the high level to the low level atTime T3, thereby turning the switch SW1 into a non-conduction state.With the switch SW1 and the switch SW2 brought into a non-conductionstate, the capacitor C11 and the capacitor C12 turn into a floatingstate (an electrically floating state).

The signal S3 changes from the low level to the high level at Time T4,thereby turning the switch SW3 into a conduction state. At this time,the charge +Q11 remains stored in the one terminal of the capacitor C11,the charge −Q11 remains stored in the other terminal of the capacitorC11, the charge +Q12 remains stored in the one terminal of the capacitorC12, and the charge −Q12 remains stored in the other terminal of thecapacitor C12. Thus, a difference obtained by subtracting a potentialoutput to the output terminal OUTM from a potential output to the outputterminal OUTP is equal to a difference obtained by subtracting apotential input to the input terminal INM from a potential input to theinput terminal INP in a period from Time T1 to Time T2.

FIG. 4B shows an equivalent circuit of the semiconductor device 100which reflects the states of the switch SW1 to the switch SW3, thechopping circuit 20_1, and the chopping circuit 20_2 in a period fromTime T4 to Time T5.

In a period from Time T4 to Time T10, the switch SW1 and the switch SW2are in a non-conduction state, and the switch SW3 is in a conductionstate. The signal S4 changes from the high level to the low level andthe signal S5 changes from the low level to the high level at Time T5.In other words, in a period from Time T5 to Time T6, the first terminaland the fourth terminal of each of the chopping circuit 20_1 and thechopping circuit 20_2 are in a conduction state, and the second terminaland the third terminal of each of the chopping circuit 20_1 and thechopping circuit 20_2 are in a conduction state.

FIG. 4C shows an equivalent circuit of the semiconductor device 100which reflects the states of the switch SW1 to the switch SW3, thechopping circuit 20_1, and the chopping circuit 20_2 in the period fromTime T5 to Time T6.

In the period from Time T4 to Time T10, the states of the switch SW1 tothe switch SW3 stay the same, and the states of the chopping circuit20_1 and the chopping circuit 20_2 change. That is, in the period fromTime T4 to Time T5, in a period from Time T6 to Time T7, and in a periodfrom Time T8 to Time T9, the first terminal and the third terminal ofeach of the chopping circuit 20_1 and the chopping circuit 20_2 are in aconduction state and the second terminal and the fourth terminal of eachof the chopping circuit 20_1 and the chopping circuit 20_2 are in aconduction state. The semiconductor device 100 has the state of theequivalent circuit shown in FIG. 4B.

In the period from Time T5 to Time T6, in a period from Time T7 to TimeT8, and in a period from Time T9 to Time T10, the first terminal and thefourth terminal of each of the chopping circuit 20_1 and the choppingcircuit 20_2 are in a conduction state and the second terminal and thethird terminal of each of the chopping circuit 20_1 and the choppingcircuit 20_2 are in a conduction state. The semiconductor device 100 hasthe state of the equivalent circuit shown in FIG. 4C.

<Semiconductor Device>

As described above, the semiconductor device 100 performs sampling ofthe potentials input to the input terminal INP and the input terminalINM in the period from Time T1 to Time T2, and outputs the potentials tothe output terminal OUTP and the output terminal OUTM in the period fromTime T4 to Time T10. At this time, a difference obtained by subtractingthe potential output to the output terminal OUTM from the potentialoutput to the output terminal OUTP is equal to a difference obtainedfrom subtracting the potential input to the input terminal INM from thepotential input to the input terminal INP.

In the period from Time T4 to Time T5, in the period from Time T6 toTime T7, and in the period from Time T8 to Time T9, the chopping circuit20_1 electrically connects the other terminal of the capacitor C11 andthe first terminal of the amplifier 30 (to be in a conduction state),and electrically connects the other terminal of the capacitor C12 andthe second terminal of the amplifier 30. Similarly, the chopping circuit20_2 electrically connects the third terminal of the amplifier 30 andthe output terminal OUTM, and electrically connects the fourth terminalof the amplifier 30 and the output terminal OUTP (the state of theequivalent circuit shown in FIG. 4B).

In the period from Time T5 to Time T6, in the period from Time T7 toTime T8, and in the period from Time T9 to Time T10, the choppingcircuit 20_1 electrically connects the other terminal of the capacitorC11 and the second terminal of the amplifier 30 (to be in a conductionstate), and electrically connects the other terminal of the capacitorC12 and the first terminal of the amplifier 30. Similarly, the choppingcircuit 20_2 electrically connects the third terminal of the amplifier30 and the output terminal OUTP, and electrically connects the fourthterminal of the amplifier 30 and the output terminal OUTM (the state ofthe equivalent circuit shown in FIG. 4C).

In other words, the semiconductor device 100 alternately has the stateof the equivalent circuit shown in FIG. 4B and the state of theequivalent circuit shown in FIG. 4C, enabling an effect of offsetvoltage attributable to the amplifier 30 to be canceled, for example. Inaddition, by replacing the polarities of the input terminals of theamplifier 30, for example, an effect of 1/f noise, thermal noise, or thelike attributable to the amplifier 30 on an output can be reduced.

The semiconductor device 100, in which transistors with a smalloff-state current such as OS transistors are used as the transistor 11to the transistor 13 and the transistor 21 to the transistor 24, canhold the potentials that are subjected to sampling in the period fromTime T1 to Time T2 for a long time.

With the use of the semiconductor device 100, a highly accurateamplifier with a reduced effect of offset voltage or noise attributableto the amplifier 30 can be achieved, and the sampled potentials can beheld for a long time; thus, the semiconductor device 100 is suitable forhighly accurate measurement such as a sensor with high output impedance,for example.

Note that this embodiment can be implemented in appropriate combinationwith the other embodiments described in this specification.

Embodiment 2

In this embodiment, structure examples of the transistor included in thesemiconductor device 100 described in the above embodiment will bedescribed. This embodiment has a structure where a layer including an OStransistor is provided to be stacked above a layer including a Sitransistor formed on a single crystal silicon substrate.

<Structure Example of Semiconductor Device>

A semiconductor device shown in FIG. 5 includes a transistor 300, atransistor 500, and a capacitive element 600. FIG. 6A is across-sectional view of the transistor 500 in the channel lengthdirection, FIG. 6B is a cross-sectional view of the transistor 500 inthe channel width direction, and FIG. 6C is a cross-sectional view ofthe transistor 300 in the channel width direction.

For example, the transistor 500 corresponds to the transistor 21described in the above embodiment, and the transistor 500 includes asecond gate (also referred to as a bottom gate or a back gate) inaddition to a first gate (also referred to as a top gate or a frontgate). Furthermore, the transistor 300 corresponds to a Si transistorincluded in the amplifier 30, and the capacitive element 600 correspondsto, for example, the capacitor C11.

The transistor 500 is a transistor including a metal oxide in itschannel-formation region (an OS transistor). The transistor 500 has anextremely small off-state current; in the above embodiment, thetransistor 500 is used for the switch SW1 to the switch SW3 and thechopping circuit 20, enabling the semiconductor device 100 to hold thesampled potential for a long time.

As shown in FIG. 5, in the semiconductor device described in thisembodiment, the transistor 500 is provided above the transistor 300, andthe capacitive element 600 is provided above the transistor 300 and thetransistor 500.

The transistor 300 is provided on a substrate 311 and includes aconductor 316, an insulator 315, a semiconductor region 313 that is apart of the substrate 311, and a low-resistance region 314 a and alow-resistance region 314 b functioning as a source region and a drainregion.

As shown in FIG. 6C, in the transistor 300, a top surface and a sidesurface in the channel width direction of the semiconductor region 313are covered with the conductor 316 with the insulator 315 therebetween.Such a Fin-type transistor 300 can have an increased effective channelwidth, and thus the transistor 300 can have improved on-statecharacteristics. In addition, since contribution of an electric field ofa gate electrode can be increased, the off-state characteristics of thetransistor 300 can be improved.

Note that the transistor 300 can be either a p-channel transistor or ann-channel transistor.

It is preferable that a region of the semiconductor region 313 where achannel is formed, a region in the vicinity thereof, the low-resistanceregion 314 a and the low-resistance region 314 b functioning as thesource region and the drain region, and the like contain a semiconductorsuch as a silicon-based semiconductor, further preferably single crystalsilicon. Alternatively, these regions may be formed using a materialcontaining Ge (germanium), SiGe (silicon germanium), GaAs (galliumarsenide), GaAlAs (gallium aluminum arsenide), or the like. A structureusing silicon whose effective mass is controlled by applying stress tothe crystal lattice and changing the lattice spacing may be employed.Alternatively, the transistor 300 may be an HEMT (High Electron MobilityTransistor) with GaAs and GaAlAs, or the like.

The low-resistance region 314 a and the low-resistance region 314 bcontain an element that imparts n-type conductivity, such as arsenic orphosphorus, or an element that imparts p-type conductivity, such asboron, in addition to the semiconductor material used for thesemiconductor region 313.

The conductor 316 functioning as a gate electrode can be formed using asemiconductor material such as silicon containing an element thatimparts n-type conductivity, such as arsenic or phosphorus, or anelement that imparts p-type conductivity, such as boron, or using aconductive material such as a metal material, an alloy material, or ametal oxide material.

Note that since the work function of a conductor depends on a materialof the conductor, Vth of the transistor can be adjusted by changing thematerial of the conductor. Specifically, it is preferable to use amaterial such as titanium nitride or tantalum nitride for the conductor.Moreover, in order to ensure both conductivity and embeddability, it ispreferable to use stacked layers of metal materials such as tungsten andaluminum for the conductor, and it is particularly preferable to usetungsten in terms of heat resistance.

Note that the transistor 300 shown in FIG. 5 is an example and is notlimited to the structure shown therein; an appropriate transistor isused in accordance with a circuit structure or a driving method.

An insulator 320, an insulator 322, an insulator 324, and an insulator326 are stacked in this order to cover the transistor 300.

The insulator 320, the insulator 322, the insulator 324, and theinsulator 326 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, or aluminum nitride.

The insulator 322 may have a function of a planarization film forplanarizing a level difference caused by the transistor 300 or the likeprovided below the insulator 322. For example, the top surface of theinsulator 322 may be planarized by planarization treatment using achemical mechanical polishing (CMP) method or the like to improveplanarity.

The insulator 324 is preferably formed using a film having a barrierproperty that prevents diffusion of hydrogen or impurities from thesubstrate 311, the transistor 300, or the like into the region where thetransistor 500 is provided.

For the film having a barrier property against hydrogen, silicon nitrideformed by a CVD method can be used, for example. Here, diffusion ofhydrogen to a semiconductor element including an oxide semiconductor,such as the transistor 500, degrades the characteristics of thesemiconductor element in some cases. Therefore, a film that inhibitshydrogen diffusion is preferably used between the transistor 500 and thetransistor 300. The film that inhibits hydrogen diffusion isspecifically a film from which a small amount of hydrogen is released.

The amount of released hydrogen can be measured by thermal desorptionspectroscopy (TDS), for example. The amount of hydrogen released fromthe insulator 324 that is converted into hydrogen atoms per area of theinsulator 324 is less than or equal to 10×10¹⁵ atoms/cm², preferablyless than or equal to 5×10¹⁵ atoms/cm², in the TDS analysis in afilm-surface temperature range of 50° C. to 500° C., for example.

Note that the dielectric constant of the insulator 326 is preferablylower than that of the insulator 324. For example, the dielectricconstant of the insulator 326 is preferably lower than 4, furtherpreferably lower than 3. The dielectric constant of the insulator 326is, for example, preferably 0.7 times or less, further preferably 0.6times or less the dielectric constant of the insulator 324. When amaterial with a low dielectric constant is used for an interlayer film,the parasitic capacitance generated between wirings can be reduced.

A conductor 328, a conductor 330, and the like that are connected to thecapacitive element 600 or the transistor 500 are embedded in theinsulator 320, the insulator 322, the insulator 324, and the insulator326. Note that the conductor 328 and the conductor 330 have a functionof a plug or a wiring. A plurality of conductors functioning as plugs orwirings are collectively denoted by the same reference numeral in somecases. Moreover, in this specification and the like, a wiring and a plugconnected to the wiring may be a single component. That is, there arecases where a part of a conductor functions as a wiring and another partof the conductor functions as a plug.

As a material for each of plugs and wirings (the conductor 328, theconductor 330, and the like), a single layer or stacked layers of aconductive material such as a metal material, an alloy material, a metalnitride material, or a metal oxide material can be used. It ispreferable to use a high-melting-point material that has both heatresistance and conductivity, such as tungsten or molybdenum, and it ispreferable to use tungsten. Alternatively, it is preferable to use alow-resistance conductive material such as aluminum or copper. The useof a low-resistance conductive material can reduce wiring resistance.

A wiring layer may be provided over the insulator 326 and the conductor330. For example, in FIG. 5, an insulator 350, an insulator 352, and aninsulator 354 are provided to be stacked in this order. Furthermore, aconductor 356 is formed in the insulator 350, the insulator 352, and theinsulator 354. The conductor 356 has a function of a plug or a wiringthat is connected to the transistor 300. Note that the conductor 356 canbe provided using a material similar to that for the conductor 328 andthe conductor 330.

For example, like the insulator 324, the insulator 350 is preferablyformed using an insulator having a barrier property against hydrogen.The conductor 356 preferably contains a conductor having a barrierproperty against hydrogen. In particular, the conductor having a barrierproperty against hydrogen is formed in an opening portion of theinsulator 350 having a barrier property against hydrogen. With thisstructure, the transistor 300 and the transistor 500 can be separated bythe barrier layer, so that diffusion of hydrogen from the transistor 300into the transistor 500 can be inhibited.

For the conductor having a barrier property against hydrogen, tantalumnitride is preferably used, for example. In addition, the use of a stackincluding tantalum nitride and tungsten, which has high conductivity,can inhibit diffusion of hydrogen from the transistor 300 while theconductivity of a wiring is maintained. In that case, a structure ispreferable in which a tantalum nitride layer having a barrier propertyagainst hydrogen is in contact with the insulator 350 having a barrierproperty against hydrogen.

A wiring layer may be provided over the insulator 354 and the conductor356. For example, in FIG. 5, an insulator 360, an insulator 362, and aninsulator 364 are provided to be stacked in this order. Furthermore, aconductor 366 is formed in the insulator 360, the insulator 362, and theinsulator 364. The conductor 366 has a function of a plug or a wiring.Note that the conductor 366 can be provided using a material similar tothat for the conductor 328 and the conductor 330.

For example, like the insulator 324, the insulator 360 is preferablyformed using an insulator having a barrier property against hydrogen.Furthermore, the conductor 366 preferably contains a conductor having abarrier property against hydrogen. In particular, the conductor having abarrier property against hydrogen is formed in an opening portion of theinsulator 360 having a barrier property against hydrogen. With thisstructure, the transistor 300 and the transistor 500 can be separated bythe barrier layer, so that diffusion of hydrogen from the transistor 300into the transistor 500 can be inhibited.

A wiring layer may be provided over the insulator 364 and the conductor366. For example, in FIG. 5, an insulator 370, an insulator 372, and aninsulator 374 are provided to be stacked in this order. Furthermore, aconductor 376 is formed in the insulator 370, the insulator 372, and theinsulator 374. The conductor 376 has a function of a plug or a wiring.Note that the conductor 376 can be provided using a material similar tothe materials for the conductor 328 and the conductor 330.

For example, like the insulator 324, the insulator 370 is preferablyformed using an insulator having a barrier property against hydrogen.Furthermore, the conductor 376 preferably contains a conductor having abarrier property against hydrogen. In particular, the conductor having abarrier property against hydrogen is formed in an opening portion of theinsulator 370 having a barrier property against hydrogen. With thisstructure, the transistor 300 and the transistor 500 can be separated bythe barrier layer, so that diffusion of hydrogen from the transistor 300into the transistor 500 can be inhibited.

A wiring layer may be provided over the insulator 374 and the conductor376. For example, in FIG. 5, an insulator 380, an insulator 382, and aninsulator 384 are provided to be stacked in this order. Furthermore, aconductor 386 is formed in the insulator 380, the insulator 382, and theinsulator 384. The conductor 386 has a function of a plug or a wiring.Note that the conductor 386 can be provided using a material similar tothe materials for the conductor 328 and the conductor 330.

For example, like the insulator 324, the insulator 380 is preferablyformed using an insulator having a barrier property against hydrogen.Furthermore, the conductor 386 preferably contains a conductor having abarrier property against hydrogen. In particular, the conductor having abarrier property against hydrogen is formed in an opening portion of theinsulator 380 having a barrier property against hydrogen. With thisstructure, the transistor 300 and the transistor 500 can be separated bythe barrier layer, so that diffusion of hydrogen from the transistor 300into the transistor 500 can be inhibited.

Although the wiring layer including the conductor 356, the wiring layerincluding the conductor 366, the wiring layer including the conductor376, and the wiring layer including the conductor 386 are describedabove, the semiconductor device of this embodiment is not limitedthereto. Three or less wiring layers that are similar to the wiringlayer including the conductor 356 may be provided, or five or morewiring layers that are similar to the wiring layer including theconductor 356 may be provided.

An insulator 510, an insulator 512, an insulator 514, and an insulator516 are provided to be stacked in this order over the insulator 384. Asubstance having a barrier property against oxygen or hydrogen ispreferably used for any of the insulator 510, the insulator 512, theinsulator 514, and the insulator 516.

The insulator 510 and the insulator 514 are preferably formed using, forexample, a film having a barrier property that prevents diffusion ofhydrogen or impurities from the substrate 311, the region where thetransistor 300 is provided, or the like into the region where thetransistor 500 is provided. Thus, a material similar to that for theinsulator 324 can be used.

For the film having a barrier property against hydrogen, silicon nitrideformed by a CVD method can be used, for example. Here, diffusion ofhydrogen to a semiconductor element including an oxide semiconductor,such as the transistor 500, degrades the characteristics of thesemiconductor element in some cases. Therefore, a film that inhibitshydrogen diffusion is preferably used between the transistor 500 and thetransistor 300. The film that inhibits hydrogen diffusion isspecifically a film from which a small amount of hydrogen is released.

For the film having a barrier property against hydrogen used as theinsulator 510 and the insulator 514, for example, a metal oxide such asaluminum oxide, hafnium oxide, or tantalum oxide is preferably used.

In particular, aluminum oxide has a high blocking effect that inhibitsthe passage of both oxygen and impurities such as hydrogen and moisturewhich are factors of a change in electrical characteristics of thetransistor. Thus, aluminum oxide can prevent the entry of impuritiessuch as hydrogen and moisture into the transistor 500 in the fabricationprocess and after the fabrication of the transistor. In addition,release of oxygen from the oxide included in the transistor 500 can beinhibited. Therefore, aluminum oxide is suitably used for a protectivefilm of the transistor 500.

The insulator 512 and the insulator 516 can be formed using a materialsimilar to that for the insulator 320, for example. When a material witha relatively low permittivity is used for the interlayer film, theparasitic capacitance between wirings can be reduced. Silicon oxidefilms, silicon oxynitride films, or the like can be used as theinsulator 512 and the insulator 516, for example.

A conductor 518, a conductor included in the transistor 500 (a conductor503), and the like are embedded in the insulator 510, the insulator 512,the insulator 514, and the insulator 516. Note that the conductor 518has a function of a plug or a wiring that is connected to the capacitiveelement 600 or the transistor 300. The conductor 518 can be providedusing a material similar to the materials for the conductor 328 and theconductor 330.

In particular, the conductor 518 in a region in contact with theinsulator 510 and the insulator 514 is preferably a conductor having abarrier property against oxygen, hydrogen, and water. With thisstructure, the transistor 300 and the transistor 500 can be separated bythe layer having a barrier property against oxygen, hydrogen, and water;thus, the diffusion of hydrogen from the transistor 300 into thetransistor 500 can be inhibited.

The transistor 500 is provided above the insulator 516.

As shown in FIG. 6A and FIG. 6B, the transistor 500 includes theconductor 503 positioned to be embedded in the insulator 514 and theinsulator 516; an insulator 520 positioned over the insulator 516 andthe conductor 503; an insulator 522 positioned over the insulator 520;an insulator 524 positioned over the insulator 522; an oxide 530 apositioned over the insulator 524; an oxide 530 b positioned over theoxide 530 a; a conductor 542 a and a conductor 542 b positioned apartfrom each other over the oxide 530 b; an insulator 580 that ispositioned over the conductor 542 a and the conductor 542 b and isprovided with an opening formed to overlap with a region between theconductor 542 a and the conductor 542 b; a conductor 560 positioned inthe opening; an insulator 550 positioned between the conductor 560 andthe oxide 530 b, the conductor 542 a, the conductor 542 b, and theinsulator 580; and an oxide 530 c positioned between the insulator 550and the oxide 530 b, the conductor 542 a, the conductor 542 b, and theinsulator 580.

As shown in FIG. 6A and FIG. 6B, an insulator 544 is preferablypositioned between the insulator 580 and the oxide 530 a, the oxide 530b, the conductor 542 a, and the conductor 542 b. In addition, as shownin FIG. 6A and FIG. 6B, the conductor 560 preferably includes aconductor 560 a provided inside the insulator 550 and a conductor 560 bprovided to be embedded inside the conductor 560 a. As shown in FIG. 6Aand FIG. 6B, an insulator 574 is preferably positioned over theinsulator 580, the conductor 560, and the insulator 550.

Hereinafter, the oxide 530 a, the oxide 530 b, and the oxide 530 c maybe collectively referred to as an oxide 530. The conductor 542 a and theconductor 542 b may be collectively referred to as a conductor 542.

Note that the transistor 500 having a structure in which three layers ofthe oxide 530 a, the oxide 530 b, and the oxide 530 c are stacked in theregion where the channel is formed and its vicinity is illustrated;however, the present invention is not limited thereto. For example, asingle layer of the oxide 530 b, a two-layer structure of the oxide 530b and the oxide 530 a, a two-layer structure of the oxide 530 b and theoxide 530 c, or a stacked-layer structure of four or more layers may beemployed. Furthermore, although the conductor 560 having a stacked-layerstructure of two layers in the transistor 500 is illustrated, thepresent invention is not limited thereto. For example, the conductor 560may have a single-layer structure or a stacked-layer structure of threeor more layers. The transistor 500 shown in FIG. 5, FIG. 6A, and FIG. 6Bis an example, and the structure is not limited thereto; an appropriatetransistor can be used in accordance with a circuit structure or adriving method.

Here, the conductor 560 functions as a gate electrode of the transistor,and the conductor 542 a and the conductor 542 b function as a sourceelectrode and a drain electrode. As described above, the conductor 560is formed to be embedded in the opening of the insulator 580 and theregion between the conductor 542 a and the conductor 542 b. Thepositions of the conductor 560, the conductor 542 a, and the conductor542 b are selected in a self-aligned manner with respect to the openingof the insulator 580. That is, in the transistor 500, the gate electrodecan be positioned between the source electrode and the drain electrodein a self-aligned manner. Thus, the conductor 560 can be formed withoutan alignment margin, resulting in a reduction in the area occupied bythe transistor 500. Accordingly, miniaturization and high integration ofthe semiconductor device can be achieved.

In addition, since the conductor 560 is formed in the region between theconductor 542 a and the conductor 542 b in a self-aligned manner, theconductor 560 does not include a region overlapping with the conductor542 a or the conductor 542 b. Thus, parasitic capacitance formed betweenthe conductor 560 and each of the conductor 542 a and the conductor 542b can be reduced. As a result, the switching speed of the transistor 500can be improved, and the transistor 500 can have high frequencycharacteristics.

The conductor 560 sometimes functions as a first gate electrode. Inaddition, the conductor 503 sometimes functions as a second gateelectrode. In that case, Vth of the transistor 500 can be controlled bychanging a potential applied to the conductor 503 independently of apotential applied to the conductor 560. In particular, Vth of thetransistor 500 can be higher than 0 V and the off-state current can bereduced by applying a negative potential to the conductor 503. Thus, adrain current at the time when a potential applied to the conductor 560is 0 V can be lower in the case where a negative potential is applied tothe conductor 503 than in the case where a negative potential is notapplied to the conductor 503.

The conductor 503 is positioned to overlap with the oxide 530 and theconductor 560. Thus, when potentials are applied to the conductor 560and the conductor 503, an electric field generated from the conductor560 and an electric field generated from the conductor 503 areconnected, so that the channel-formation region formed in the oxide 530can be covered. In this specification and the like, a transistorstructure in which a channel-formation region is electrically surroundedby electric fields of a first gate electrode and a second gate electrodeis referred to as a surrounded channel (S-channel) structure.

Furthermore, in this specification and the like, the S-channel structurehas a feature that the side surface and the vicinity of the oxide 530 incontact with the conductor 542 a and the conductor 542 b functioning asthe source electrode and the drain electrode are of i-type like thechannel-formation region. The side surface and the vicinity of the oxide530 in contact with the conductor 542 a and the conductor 542 b are incontact with the insulator 544 and thus can be of i-type like thechannel-formation region. Note that in this specification and the like,“i-type” can be equated with “highly purified intrinsic” to be describedlater. The S-channel structure disclosed in this specification and thelike is different from a Fin-type structure and a planar structure. Withthe S-channel structure, resistance to a short-channel effect can beenhanced, that is, a transistor in which a short-channel effect is lesslikely to occur can be provided.

The conductor 503 has a structure similar to that of the conductor 518;a conductor 503 a is formed in contact with an inner wall of an openingin the insulator 514 and the insulator 516, and a conductor 503 b isformed on the inner side.

The insulator 520, the insulator 522, the insulator 524, and theinsulator 550 each have a function of a gate insulating film.

Here, as the insulator 524 in contact with the oxide 530, an insulatorthat contains oxygen more than oxygen in the stoichiometric compositionis preferably used. That is, an excess-oxygen region is preferablyformed in the insulator 524. When such an insulator containing excessoxygen is provided in contact with the oxide 530, oxygen vacancies inthe oxide 530 can be reduced and the reliability of the transistor 500can be improved.

As the insulator including an excess-oxygen region, specifically, anoxide material from which part of oxygen is released by heating ispreferably used. An oxide from which oxygen is released by heating is anoxide film in which the amount of released oxygen converted into oxygenatoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greaterthan or equal to 1.0×10¹⁹ atoms/cm³, further preferably greater than orequal to 2.0×10¹⁹ atoms/cm³ or greater than or equal to 3.0×10²⁰atoms/cm³ in TDS (Thermal Desorption Spectroscopy) analysis. Note thatthe temperature of the film surface in the TDS analysis is preferablywithin the range of 100° C. to 700° C., or 100° C. to 400° C.

When the insulator 524 includes an excess-oxygen region, it ispreferable that the insulator 522 have a function of inhibitingdiffusion of oxygen (e.g., oxygen atoms and oxygen molecules) (or thatthe above oxygen be less likely to pass through the insulator 522).

When the insulator 522 has a function of inhibiting diffusion of oxygenor impurities, oxygen contained in the oxide 530 is not diffused to theinsulator 520 side, which is preferable. Furthermore, the conductor 503can be inhibited from reacting with oxygen contained in the insulator524 or the oxide 530.

For example, the insulator 522 is preferably formed using a single layeror stacked layers of an insulator containing aluminum oxide, hafniumoxide, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT),strontium titanate (SrTiO₃), (Ba,Sr)TiO₃ (BST), or the like. Withminiaturization and high integration of transistors, a problem such asleakage current may arise because of a thinner gate insulating film.When a high-k material is used for the insulator functioning as the gateinsulating film, a gate potential at the time when the transistoroperates can be reduced while the physical thickness is maintained.

It is particularly preferable to use an insulator containing an oxide ofone or both of aluminum and hafnium, which is an insulating materialhaving a function of inhibiting diffusion of impurities, oxygen, and thelike (through which the above oxygen is less likely to pass). As theinsulator containing an oxide of one or both of aluminum and hafnium,aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium(hafnium aluminate), or the like is preferably used. In the case wherethe insulator 522 is formed using such a material, the insulator 522functions as a layer that inhibits release of oxygen from the oxide 530and mixing of impurities such as hydrogen from the periphery of thetransistor 500 into the oxide 530.

Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobiumoxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, orzirconium oxide may be added to these insulators, for example.Alternatively, these insulators may be subjected to nitriding treatment.Silicon oxide, silicon oxynitride, or silicon nitride may be stackedover the insulator.

It is preferable that the insulator 520 be thermally stable. Forexample, silicon oxide and silicon oxynitride, which have thermalstability, are preferable. Furthermore, when an insulator that is ahigh-k material is combined with silicon oxide or silicon oxynitride,the insulator 520 having a stacked-layer structure that has thermalstability and a high dielectric constant can be obtained.

Note that the insulator 520, the insulator 522, and the insulator 524may each have a stacked-layer structure of two or more layers. In suchcases, without limitation to a stacked-layer structure formed of thesame material, a stacked-layer structure formed of different materialsmay be employed.

In the transistor 500, a metal oxide functioning as an oxidesemiconductor is preferably used as the oxide 530 including thechannel-formation region. For example, as the oxide 530, a metal oxidesuch as an In-M-Zn oxide (the element M is one or more kinds selectedfrom aluminum, gallium, yttrium, copper, vanadium, beryllium, boron,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like)is preferably used. Furthermore, as the oxide 530, an In—Ga oxide or anIn—Zn oxide may be used.

Note that the metal oxide functioning as an oxide semiconductor may beformed by a sputtering method or an ALD (Atomic Layer Deposition)method. The metal oxide functioning as an oxide semiconductor will bedescribed in another embodiment.

Furthermore, a metal oxide with a low carrier density is preferably usedin the transistor 500. In order to reduce the carrier density of themetal oxide, the concentration of impurities in the metal oxide isreduced so that the density of defect states can be reduced. In thisspecification and the like, a state with a low impurity concentrationand a low density of defect states is referred to as a highly purifiedintrinsic or substantially highly purified intrinsic state. Examples ofimpurities in a metal oxide include hydrogen, nitrogen, alkali metal,alkaline earth metal, iron, nickel, and silicon.

In particular, hydrogen contained in a metal oxide reacts with oxygenbonded to a metal atom to be water, and thus forms oxygen vacancies inthe metal oxide in some cases. If the channel-formation region in themetal oxide includes oxygen vacancies, the transistor sometimes hasnormally-on characteristics. In some cases, a defect that is an oxygenvacancy into which hydrogen has entered functions as a donor andgenerates an electron serving as a carrier. In other cases, bonding ofpart of hydrogen to oxygen bonded to a metal atom generates electronsserving as carriers. Thus, a transistor using a metal oxide containing alarge amount of hydrogen is likely to have normally-on characteristics.

A defect that is an oxygen vacancy into which hydrogen has entered canfunction as a donor of a metal oxide. However, it is difficult toevaluate the defects quantitatively. Thus, the metal oxide is sometimesevaluated by not its donor concentration but its carrier density.Therefore, in this specification and the like, as the parameter of themetal oxide, the carrier density assuming the state where an electricfield is not applied is sometimes used instead of the donorconcentration. That is, “carrier density” in this specification and thelike can be replaced with “donor concentration” in some cases.

Consequently, when a metal oxide is used for the oxide 530, hydrogen inthe metal oxide is preferably reduced as much as possible. Specifically,the hydrogen concentration of the metal oxide, which is measured bysecondary ion mass spectrometry (SIMS), is lower than 1×10²⁰ atoms/cm³,preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than5×10¹⁸ atoms/cm³, and still further preferably lower than 1×10¹⁸atoms/cm³. When a metal oxide with a sufficiently low concentration ofimpurities such as hydrogen is used for a channel-formation region of atransistor, the transistor can have stable electrical characteristics.

When a metal oxide is used for the oxide 530, the carrier density of themetal oxide in the channel-formation region is preferably lower than orequal to 1×10¹⁸ cm⁻³, further preferably lower than 1×10¹⁷ cm⁻³, stillfurther preferably lower than 1×10¹⁶ cm⁻³, yet further preferably lowerthan 1×10¹³ cm⁻³, and yet still further preferably lower than 1×10¹²cm⁻³. Note that the lower limit of the carrier density of the metaloxide in the channel-formation region is not particularly limited andcan be, for example, 1×10⁻⁹ cm⁻³.

When a metal oxide is used for the oxide 530, contact between theconductor 542 (the conductor 542 a and the conductor 542 b) and theoxide 530 may make oxygen in the oxide 530 diffuse into the conductor542, resulting in oxidation of the conductor 542. It is highly possiblethat oxidation of the conductor 542 lowers the conductivity of theconductor 542. Note that diffusion of oxygen in the oxide 530 into theconductor 542 can be interpreted as absorption of oxygen in the oxide530 by the conductor 542.

When oxygen in the oxide 530 is diffused into the conductor 542 (theconductor 542 a and the conductor 542 b), a layer is sometimes formedbetween the conductor 542 a and the oxide 530 b, and between theconductor 542 b and the oxide 530 b. The layer contains more oxygen thanthe conductor 542 does, and thus presumably has an insulating property.In this case, a three-layer structure of the conductor 542, the layer,and the oxide 530 b can be regarded as a three-layer structure of ametal, an insulator, and a semiconductor and is sometimes referred to asa MIS (Metal-Insulator-Semiconductor) structure or a diode junctionstructure having an MIS structure as its main part.

The above layer is not necessarily formed between the conductor 542 andthe oxide 530 b, but the layer may be formed between the conductor 542and the oxide 530 c, or formed between the conductor 542 and the oxide530 b and between the conductor 542 and the oxide 530 c.

The metal oxide functioning as the channel-formation region in the oxide530 has a bandgap of preferably 2 eV or larger, further preferably 2.5eV or larger. With the use of a metal oxide having such a wide bandgap,the off-state current of the transistor can be reduced.

Semiconductor materials that can be used for the oxide 530 are notlimited to the above metal oxides. A semiconductor material having abandgap (a semiconductor material that is not a zero-gap semiconductor)can be used for the oxide 530. For example, a single elementsemiconductor such as silicon, a compound semiconductor such as galliumarsenide, or a layered material (also referred to as an atomic layeredmaterial or a two-dimensional material) is preferably used as asemiconductor material. In particular, a layered material functioning asa semiconductor is preferably used as a semiconductor material.

Here, in this specification and the like, the layered material is ageneral term of a group of materials having a layered crystal structure.In the layered crystal structure, layers formed by covalent bonding orionic bonding are stacked with bonding such as the Van der Waals force,which is weaker than covalent bonding or ionic bonding. The layeredmaterial has high electrical conductivity in a monolayer, that is, hightwo-dimensional electrical conductivity. When a material functioning asa semiconductor and having high two-dimensional electrical conductivityis used for a channel-formation region, a transistor having a highon-state current can be provided.

Examples of the layered material include graphene, silicene, andchalcogenide. Chalcogenide is a compound containing chalcogen. Chalcogenis a general term of elements belonging to Group 16, which includesoxygen, sulfur, selenium, tellurium, polonium, and livermorium. Examplesof chalcogenide include transition metal chalcogenide and chalcogenideof Group 13 elements.

As the oxide 530, a transition metal chalcogenide functioning as asemiconductor is preferably used, for example. Specific examples of thetransition metal chalcogenide which can be used for the oxide 530include molybdenum sulfide (typically MoS₂), molybdenum selenide(typically MoSe₂), molybdenum telluride (typically MoTe₂), tungstensulfide (typically WS₂), tungsten selenide (typically WSe₂), tungstentelluride (typically WTe₂), hafnium sulfide (typically HfS₂), hafniumselenide (typically HfSe₂), zirconium sulfide (typically ZrS₂), andzirconium selenide (typically ZrSe₂).

When the oxide 530 includes the oxide 530 a under the oxide 530 b, it ispossible to inhibit diffusion of impurities into the oxide 530 b fromthe components formed below the oxide 530 a. Moreover, including theoxide 530 c over the oxide 530 b makes it possible to inhibit diffusionof impurities into the oxide 530 b from the components formed above theoxide 530 c.

Note that the oxide 530 preferably has a stacked-layer structure of aplurality of oxide layers that differ in the atomic ratio of metalatoms. Specifically, the atomic proportion of the element M in theconstituent elements in the metal oxide used as the oxide 530 a ispreferably higher than the atomic proportion of the element Min theconstituent elements in the metal oxide used as the oxide 530 b. Inaddition, the atomic ratio of the element M to In in the metal oxideused as the oxide 530 a is preferably higher than the atomic ratio ofthe element M to In in the metal oxide used as the oxide 530 b.Furthermore, the atomic ratio of In to the element M in the metal oxideused as the oxide 530 b is preferably higher than the atomic ratio of Into the element

Min the metal oxide used as the oxide 530 a. A metal oxide that can beused as the oxide 530 a or the oxide 530 b can be used as the oxide 530c.

The energy of the conduction band minimum of each of the oxide 530 a andthe oxide 530 c is preferably higher than the energy of the conductionband minimum of the oxide 530 b.

In other words, the electron affinity of each of the oxide 530 a and theoxide 530 c is preferably smaller than the electron affinity of theoxide 530 b.

The energy level of the conduction band minimum gradually changes atjunction portions of the oxide 530 a, the oxide 530 b, and the oxide 530c. In other words, the energy level of the conduction band minimum atthe junction portions of the oxide 530 a, the oxide 530 b, and the oxide530 c continuously changes or is continuously connected. To obtain this,the density of defect states in a mixed layer formed at the interfacebetween the oxide 530 a and the oxide 530 b and the interface betweenthe oxide 530 b and the oxide 530 c is preferably made low.

Specifically, when the oxide 530 a and the oxide 530 b or the oxide 530b and the oxide 530 c contain a common element (as a main component) inaddition to oxygen, a mixed layer with a low density of defect statescan be formed. For example, in the case where the oxide 530 b is anIn—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or thelike is preferably used as the oxide 530 a and the oxide 530 c.

At this time, the oxide 530 b serves as a main carrier path. When theoxide 530 a and the oxide 530 c have the above structure, the density ofdefect states at the interface between the oxide 530 a and the oxide 530b and the interface between the oxide 530 b and the oxide 530 c can bemade low. Thus, the influence of interface scattering on carrierconduction is small, and the transistor 500 can have a high on-statecurrent.

The conductor 542 (the conductor 542 a and the conductor 542 b)functioning as the source electrode and the drain electrode is providedover the oxide 530 b. For the conductor 542, it is preferable to use ametal element selected from aluminum, chromium, copper, silver, gold,platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium,vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium,ruthenium, iridium, strontium, and lanthanum; an alloy containing any ofthe above metal elements; an alloy containing a combination of the abovemetal elements; or the like. For example, it is preferable to usetantalum nitride, titanium nitride, tungsten, a nitride containingtitanium and aluminum, a nitride containing tantalum and aluminum,ruthenium oxide, ruthenium nitride, an oxide containing strontium andruthenium, an oxide containing lanthanum and nickel, or the like.Tantalum nitride, titanium nitride, a nitride containing titanium andaluminum, a nitride containing tantalum and aluminum, ruthenium oxide,ruthenium nitride, an oxide containing strontium and ruthenium, and anoxide containing lanthanum and nickel are preferable because they areoxidation-resistant conductive materials or materials that retain theirconductivity even after absorbing oxygen.

As shown in FIG. 6A, a region 543 (a region 543 a and a region 543 b) issometimes formed as a low-resistance region at and near the interfacebetween the oxide 530 and the conductor 542. In that case, the region543 a functions as one of a source region and a drain region, and theregion 543 b functions as the other of the source region and the drainregion. Furthermore, the channel-formation region is formed in a regionbetween the region 543 a and the region 543 b.

When the conductor 542 is provided in contact with the oxide 530, theoxygen concentration in the region 543 sometimes decreases. In addition,a metal compound layer that contains the metal contained in theconductor 542 and the component of the oxide 530 is sometimes formed inthe region 543. In such a case, the carrier density of the region 543increases, and the region 543 becomes a low-resistance region.

The insulator 544 is provided to cover the conductor 542 and inhibitsoxidation of the conductor 542. At this time, the insulator 544 may beprovided to cover a side surface of the oxide 530 and to be in contactwith the insulator 524.

A metal oxide containing one or more kinds selected from hafnium,aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum,nickel, germanium, magnesium, and the like can be used as the insulator544.

For the insulator 544, it is particularly preferable to use an insulatorcontaining an oxide of one or both of aluminum and hafnium, for example,aluminum oxide, hafnium oxide, or an oxide containing aluminum andhafnium (hafnium aluminate). In particular, hafnium aluminate has higherheat resistance than a hafnium oxide film. Therefore, hafnium aluminateis preferable because it is less likely to be crystallized by heattreatment in a later step. Note that the insulator 544 is not anessential component when the conductor 542 is an oxidation-resistantmaterial or does not significantly lose its conductivity even afterabsorbing oxygen. Design is appropriately set in consideration ofrequired transistor characteristics.

The insulator 550 functions as a gate insulating film. The insulator 550is preferably positioned in contact with the inner side (the top surfaceand the side surface) of the oxide 530 c. The insulator 550 ispreferably formed using an insulator from which oxygen is released byheating. For example, the insulator 550 is an oxide film in which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to1.0×10¹⁹ atoms/cm³, further preferably greater than or equal to 2.0×10¹⁹atoms/cm³ or greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis. Note that the temperature of the film surface in the TDSanalysis is preferably within the range of 100° C. to 700° C.

Specifically, silicon oxide containing excess oxygen, siliconoxynitride, silicon nitride oxide, silicon nitride, silicon oxide towhich fluorine is added, silicon oxide to which carbon is added, siliconoxide to which carbon and nitrogen are added, porous silicon oxide, orthe like can be used. In particular, silicon oxide and siliconoxynitride, which have thermal stability, are preferable.

When an insulator from which oxygen is released by heating is providedas the insulator 550 in contact with the top surface of the oxide 530 c,oxygen can be effectively supplied from the insulator 550 to thechannel-formation region of the oxide 530 b through the oxide 530 c.Furthermore, as in the insulator 524, the concentration of impuritiessuch as water or hydrogen in the insulator 550 is preferably reduced.The thickness of the insulator 550 is preferably greater than or equalto 1 nm and less than or equal to 20 nm.

To efficiently supply excess oxygen contained in the insulator 550 tothe oxide 530, a metal oxide may be provided between the insulator 550and the conductor 560. The metal oxide preferably inhibits diffusion ofoxygen from the insulator 550 into the conductor 560. Providing themetal oxide that inhibits diffusion of oxygen inhibits diffusion ofexcess oxygen from the insulator 550 into the conductor 560. That is, areduction in the amount of excess oxygen supplied to the oxide 530 canbe inhibited. Moreover, oxidation of the conductor 560 due to excessoxygen can be inhibited. For the metal oxide, a material that can beused for the insulator 544 is used.

Although the conductor 560 functioning as the first gate electrode has atwo-layer structure in FIG. 6A and FIG. 6B, the conductor 560 may have asingle-layer structure or a stacked-layer structure of three or morelayers.

For the conductor 560 a, it is preferable to use a conductive materialhaving a function of inhibiting diffusion of impurities such as ahydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, anitrogen molecule, a nitrogen oxide molecule (N₂O, NO, NO₂, and thelike), and a copper atom. Alternatively, it is preferable to use aconductive material having a function of inhibiting diffusion of oxygen(e.g., at least one of an oxygen atom, an oxygen molecule, and thelike). When the conductor 560 a has a function of inhibiting diffusionof oxygen, it is possible to inhibit a reduction in conductivity of theconductor 560 b due to oxidation caused by oxygen contained in theinsulator 550. As a conductive material having a function of inhibitingdiffusion of oxygen, for example, tantalum, tantalum nitride, ruthenium,ruthenium oxide, or the like is preferably used.

The conductor 560 b is preferably formed using a conductive materialcontaining tungsten, copper, or aluminum as its main component.Furthermore, the conductor 560 b also functions as a wiring and thus ispreferably a conductor having high conductivity. For example, aconductive material containing tungsten, copper, or aluminum as its maincomponent can be used. The conductor 560 b can have a stacked-layerstructure, for example, a stacked-layer structure of any of the aboveconductive materials and titanium or titanium nitride.

The insulator 580 is provided over the conductor 542 with the insulator544 therebetween. The insulator 580 preferably includes an excess-oxygenregion. For example, the insulator 580 preferably contains siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,silicon oxide to which fluorine is added, silicon oxide to which carbonis added, silicon oxide to which carbon and nitrogen are added, poroussilicon oxide, a resin, or the like. In particular, silicon oxide andsilicon oxynitride, which have thermal stability, are preferable. Inparticular, silicon oxide and porous silicon oxide are preferablebecause an excess-oxygen region can be easily formed in a later step.

When the insulator 580 from which oxygen is released by heating isprovided in contact with the oxide 530 c, oxygen in the insulator 580can be efficiently supplied to the oxide 530 through the oxide 530 c.Note that the concentration of impurities such as water or hydrogen inthe insulator 580 is preferably reduced.

The opening in the insulator 580 is formed to overlap with the regionbetween the conductor 542 a and the conductor 542 b. Accordingly, theconductor 560 is formed to be embedded in the opening in the insulator580 and the region between the conductor 542 a and the conductor 542 b.

The gate length needs to be short for miniaturization of thesemiconductor device, but it is necessary to prevent a reduction inconductivity of the conductor 560. When the conductor 560 is made thickto achieve this, the conductor 560 might have a shape with a high aspectratio. In this embodiment, the conductor 560 is provided to be embeddedin the opening in the insulator 580; thus, even when the conductor 560has a shape with a high aspect ratio, the conductor 560 can be formedwithout collapsing during the process.

The insulator 574 is preferably provided in contact with the top surfaceof the insulator 580, the top surface of the conductor 560, and the topsurface of the insulator 550. When the insulator 574 is deposited by asputtering method, an excess-oxygen region can be provided in theinsulator 550 and the insulator 580. Thus, oxygen can be supplied fromthe excess-oxygen region to the oxide 530.

As the insulator 574, a metal oxide containing one or more kindsselected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten,titanium, tantalum, nickel, germanium, magnesium, and the like can beused, for example.

In particular, aluminum oxide has a high barrier property, and even athin aluminum oxide film having a thickness greater than or equal to 0.5nm and less than or equal to 3.0 nm can inhibit diffusion of hydrogenand nitrogen. Thus, aluminum oxide deposited by a sputtering methodserves as an oxygen supply source and can also have a function of abarrier film against impurities such as hydrogen.

An insulator 581 functioning as an interlayer film is preferablyprovided over the insulator 574. As in the insulator 524 and the like,the concentration of impurities such as water or hydrogen in theinsulator 581 is preferably reduced.

A conductor 540 a and a conductor 540 b are positioned in openingsformed in the insulator 581, the insulator 574, the insulator 580, andthe insulator 544. The conductor 540 a and the conductor 540 b areprovided to face each other with the conductor 560 therebetween. Thestructures of the conductor 540 a and the conductor 540 b are similar tothe structures of a conductor 546 and a conductor 548 that will bedescribed later.

An insulator 582 is provided over the insulator 581. A substance havinga barrier property against oxygen or hydrogen is preferably used for theinsulator 582. Therefore, a material similar to that for the insulator514 can be used for the insulator 582. For the insulator 582, a metaloxide such as aluminum oxide, hafnium oxide, or tantalum oxide ispreferably used, for example.

In particular, aluminum oxide has a high blocking effect that inhibitsthe passage of both oxygen and impurities such as hydrogen and moisturewhich are factors of a change in electrical characteristics of thetransistor. Thus, aluminum oxide can prevent the entry of impuritiessuch as hydrogen and moisture into the transistor 500 in the fabricationprocess and after the fabrication of the transistor. In addition,release of oxygen from the oxide included in the transistor 500 can beinhibited. Therefore, aluminum oxide is suitably used for a protectivefilm of the transistor 500.

An insulator 586 is provided over the insulator 582. For the insulator586, a material similar to that for the insulator 320 can be used. Whena material with a relatively low permittivity is used for the interlayerfilm, the parasitic capacitance between wirings can be reduced. Asilicon oxide film, a silicon oxynitride film, or the like can be usedfor the insulator 586, for example.

The conductor 546, the conductor 548, and the like are embedded in theinsulator 520, the insulator 522, the insulator 524, the insulator 544,the insulator 580, the insulator 574, the insulator 581, the insulator582, and the insulator 586.

The conductor 546 and the conductor 548 have functions of plugs orwirings that are connected to the capacitive element 600, the transistor500, or the transistor 300. The conductor 546 and the conductor 548 canbe provided using a material similar to the materials for the conductor328 and the conductor 330.

Next, the capacitive element 600 is provided above the transistor 500.The capacitive element 600 includes a conductor 610, a conductor 620,and an insulator 630.

A conductor 612 may be provided over the conductor 546 and the conductor548. The conductor 612 has a function of a plug or a wiring that isconnected to the transistor 500. The conductor 610 has a function of anelectrode of the capacitive element 600. The conductor 612 and theconductor 610 can be formed at the same time.

The conductor 612 and the conductor 610 can be formed using a metal filmcontaining an element selected from molybdenum, titanium, tantalum,tungsten, aluminum, copper, chromium, neodymium, and scandium; a metalnitride film containing any of the above elements as its component (atantalum nitride film, a titanium nitride film, a molybdenum nitridefilm, or a tungsten nitride film); or the like. Alternatively, it ispossible to use a conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added.

Although the conductor 612 and the conductor 610 having a single-layerstructure are illustrated in FIG. 5, the structure is not limitedthereto, and a stacked-layer structure of two or more layers may beemployed. For example, between a conductor having a barrier property anda conductor having high conductivity, a conductor that is highlyadhesive to the conductor having a barrier property and the conductorhaving high conductivity may be formed.

The conductor 620 is provided to overlap with the conductor 610 with theinsulator 630 therebetween. Note that the conductor 620 can be formedusing a conductive material such as a metal material, an alloy material,or a metal oxide material. It is preferable to use a high-melting-pointmaterial that has both heat resistance and conductivity, such astungsten or molybdenum, and it is particularly preferable to usetungsten. In addition, in the case where the conductor 620 is formedconcurrently with another component such as a conductor, Cu (copper), Al(aluminum), or the like, which is a low-resistance metal material, isused.

An insulator 650 is provided over the conductor 620 and the insulator630. The insulator 650 can be provided using a material similar to thatfor the insulator 320. The insulator 650 may function as a planarizationfilm that covers an uneven shape thereunder.

With the use of this structure, a change in electrical characteristicscan be inhibited and reliability can be improved in a semiconductordevice including an OS transistor. Alternatively, an OS transistorhaving a high on-state current can be provided. Alternatively, an OStransistor having a low off-state current can be provided.Alternatively, a semiconductor device with low power consumption can beprovided. Alternatively, a semiconductor device including an OStransistor can be miniaturized or highly integrated.

<Structure Example of Transistor>

Note that the structure of the transistor 500 in the semiconductordevice described in this embodiment is not limited to the above.Examples of structures that can be used for the transistor 500 will bedescribed below.

<Structure Example 1 of Transistor>

A structure example of a transistor 510A is described with reference toFIG. 7A, FIG. 7B, and FIG. 7C. FIG. 7A is a top view of the transistor510A. FIG. 7B is a cross-sectional view of a portion indicated by thedashed-dotted line L1-L2 in FIG. 7A. FIG. 7C is a cross-sectional viewof a portion indicated by the dashed-dotted line W1-W2 in FIG. 7A. Notethat for clarity of the drawing, some components are not illustrated inthe top view of FIG. 7A.

FIG. 7A, FIG. 7B, and FIG. 7C show the transistor 510A and the insulator511, the insulator 512, the insulator 514, the insulator 516, theinsulator 580, the insulator 582, and an insulator 584 that function asinterlayer films. In addition, the conductor 546 (a conductor 546 a anda conductor 546 b) that is electrically connected to the transistor 510Aand functions as a contact plug, and the conductor 503 functioning as awiring are illustrated.

The transistor 510A includes the conductor 560 (the conductor 560 a andthe conductor 560 b) functioning as a first gate electrode; a conductor505 (a conductor 505 a and a conductor 505 b) functioning as a secondgate electrode; the insulator 550 functioning as a first gate insulatingfilm; an insulator 521, the insulator 522, and the insulator 524 thatfunction as a second gate insulating film; the oxide 530 (the oxide 530a, the oxide 530 b, and the oxide 530 c) including a region where achannel is formed; the conductor 542 a functioning as one of a sourceand a drain; the conductor 542 b functioning as the other of the sourceand the drain; and the insulator 574.

In the transistor 510A shown in FIG. 7B, the oxide 530 c, the insulator550, and the conductor 560 are positioned in an opening portion providedin the insulator 580 with the insulator 574 therebetween. Moreover, theoxide 530 c, the insulator 550, and the conductor 560 are positionedbetween the conductor 542 a and the conductor 542 b.

The insulator 511 and the insulator 512 each function as an interlayerfilm.

As the interlayer film, a single layer or stacked layers of an insulatorsuch as silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitrideoxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafniumnitride oxide, hafnium nitride, tantalum oxide, zirconium oxide, leadzirconate titanate (PZT), strontium titanate (SrTiO₃), or (Ba,Sr)TiO₃(BST) can be used. Alternatively, aluminum oxide, bismuth oxide,germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungstenoxide, yttrium oxide, or zirconium oxide may be added to theseinsulators, for example. Alternatively, these insulators may besubjected to nitriding treatment. Silicon oxide, silicon oxynitride, orsilicon nitride may be stacked over the insulator.

For example, the insulator 511 preferably functions as a barrier filmthat inhibits entry of impurities such as water or hydrogen into thetransistor 510A from the substrate side. Accordingly, for the insulator511, it is preferable to use an insulating material that has a functionof inhibiting diffusion of impurities such as a hydrogen atom, ahydrogen molecule, a water molecule, and a copper atom (through whichthe above impurities are less likely to pass). Alternatively, it ispreferable to use an insulating material that has a function ofinhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, anoxygen molecule, and the like) (through which the above oxygen is lesslikely to pass). Moreover, aluminum oxide or silicon nitride, forexample, may be used for the insulator 511. This structure can inhibitdiffusion of impurities such as hydrogen and water to the transistor510A side from the substrate side through the insulator 511.

For example, the permittivity of the insulator 512 is preferably lowerthan that of the insulator 511. When a material with a low permittivityis used for the interlayer film, the parasitic capacitance generatedbetween wirings can be reduced.

The conductor 503 is formed to be embedded in the insulator 512. Here,the top surface of the conductor 503 and the top surface of theinsulator 512 can be substantially level with each other. Note thatalthough the conductor 503 has a single-layer structure, the presentinvention is not limited thereto. For example, the conductor 503 mayhave a multilayer structure of two or more layers. Note that for theconductor 503, a conductive material that has high conductivity andcontains tungsten, copper, or aluminum as its main component ispreferably used.

In the transistor 510A, the conductor 560 sometimes functions as a firstgate electrode. The conductor 505 sometimes functions as a second gateelectrode. In that case, the threshold voltage of the transistor 510Acan be controlled by changing a potential applied to the conductor 505independently of a potential applied to the conductor 560. Inparticular, when a negative potential is applied to the conductor 505,the threshold voltage of the transistor 510A can be higher than 0 V, andthe off-state current can be reduced. Thus, a drain current at the timewhen a potential applied to the conductor 560 is 0 V can be lower in thecase where a negative potential is applied to the conductor 505 than inthe case where a negative potential is not applied to the conductor 505.

For example, when the conductor 505 and the conductor 560 are providedto overlap with each other, in the case where a potential is applied tothe conductor 560 and the conductor 505, an electric field generatedfrom the conductor 560 and an electric field generated from theconductor 505 are connected, so that the channel-formation region formedin the oxide 530 can be covered.

That is, the channel-formation region can be electrically surrounded bythe electric field of the conductor 560 having a function of the firstgate electrode and the electric field of the conductor 505 having afunction of the second gate electrode. In other words, the transistor510A has a surrounded channel (S-channel) structure, like the transistor500 described above.

Like the insulator 511 or the insulator 512, the insulator 514 and theinsulator 516 each function as an interlayer film. For example, theinsulator 514 preferably functions as a barrier film that inhibits entryof impurities such as water or hydrogen into the transistor 510A fromthe substrate side. This structure can inhibit diffusion of impuritiessuch as hydrogen and water to the transistor 510A side from thesubstrate side through the insulator 514. Moreover, for example, theinsulator 516 preferably has a lower permittivity than the insulator514. When a material with a low permittivity is used for the interlayerfilm, the parasitic capacitance generated between wirings can bereduced.

In the conductor 505 functioning as the second gate, the conductor 505 ais formed in contact with an inner wall of an opening in the insulator514 and the insulator 516, and the conductor 505 b is formed furtherinside. Here, the top surfaces of the conductor 505 a and the conductor505 b and the top surface of the insulator 516 can be substantiallylevel with each other. Although the transistor 510A has a structure inwhich the conductor 505 a and the conductor 505 b are stacked, thepresent invention is not limited thereto. For example, the conductor 505may have a single-layer structure or a stacked-layer structure of threeor more layers.

Here, for the conductor 505 a, it is preferable to use a conductivematerial that has a function of inhibiting diffusion of impurities suchas a hydrogen atom, a hydrogen molecule, a water molecule, and a copperatom (through which the above impurities are less likely to pass).Alternatively, it is preferable to use a conductive material that has afunction of inhibiting diffusion of oxygen (e.g., at least one of anoxygen atom, an oxygen molecule, and the like) (through which the aboveoxygen is less likely to pass). Note that in this specification and thelike, a function of inhibiting diffusion of impurities or oxygen means afunction of inhibiting diffusion of any one or all of the aboveimpurities and the above oxygen.

For example, when the conductor 505 a has a function of inhibitingdiffusion of oxygen, a reduction in conductivity of the conductor 505 bdue to oxidation can be inhibited.

In the case where the conductor 505 doubles as a wiring, the conductor505 b is preferably formed using a conductive material that has highconductivity and contains tungsten, copper, or aluminum as its maincomponent. In that case, the conductor 503 is not necessarily provided.Note that the conductor 505 b is illustrated as a single layer but mayhave a stacked-layer structure, for example, a stack of any of the aboveconductive materials and titanium or titanium nitride.

The insulator 521, the insulator 522, and the insulator 524 each have afunction of a second gate insulating film.

The insulator 522 preferably has a barrier property. The insulator 522having a barrier property functions as a layer that inhibits entry ofimpurities such as hydrogen into the transistor 510A from thesurroundings of the transistor 510A.

For the insulator 522, a single layer or stacked layers of an insulatorcontaining aluminum oxide, hafnium oxide, an oxide containing aluminumand hafnium (hafnium aluminate), oxynitride containing aluminum andhafnium, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT),strontium titanate (SrTiO₃), or (Ba,Sr)TiO₃ (BST), are preferably used,for example. With miniaturization and high integration of transistors, aproblem such as leakage current may arise because of a thinner gateinsulating film. When a high-k material is used for the insulatorfunctioning as the gate insulating film, a gate potential at the timewhen the transistor operates can be reduced while the physical thicknessis maintained.

It is preferable that the insulator 521 be thermally stable. Forexample, silicon oxide and silicon oxynitride, which have thermalstability, are preferable. In addition, a combination of an insulator ofa high-k material and silicon oxide or silicon oxynitride allows theinsulator 521 to have a stacked-layer structure with thermal stabilityand a high dielectric constant.

Note that the second gate insulating film is shown to have astacked-layer structure of three layers in FIG. 7B and FIG. 7C, but mayhave a stacked-layer structure of two or less layers or four or morelayers. In such cases, without limitation to a stacked-layer structureformed of the same material, a stacked-layer structure formed ofdifferent materials may be employed.

The oxide 530 including a region functioning as the channel-formationregion includes the oxide 530 a, the oxide 530 b over the oxide 530 a,and the oxide 530 c over the oxide 530 b. Including the oxide 530 aunder the oxide 530 b makes it possible to inhibit diffusion ofimpurities into the oxide 530 b from the components formed below theoxide 530 a. Moreover, including the oxide 530 c over the oxide 530 bmakes it possible to inhibit diffusion of impurities into the oxide 530b from the components formed above the oxide 530 c. As the oxide 530,the above-described oxide semiconductor, which is one type of metaloxide, can be used.

Note that the oxide 530 c is preferably provided in the opening portionprovided in the insulator 580 with the insulator 574 therebetween. Whenthe insulator 574 has a barrier property, diffusion of impurities fromthe insulator 580 into the oxide 530 can be inhibited.

One of the conductors 542 functions as a source electrode and the otherfunctions as a drain electrode.

For the conductor 542 a and the conductor 542 b, a metal such asaluminum, titanium, chromium, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, or tungsten or an alloy containing any ofthe metals as its main component can be used. In particular, a metalnitride film of tantalum nitride or the like is preferable because ithas a barrier property against hydrogen or oxygen and high oxidationresistance.

Although a single-layer structure is illustrated in FIG. 7B, astacked-layer structure of two or more layers may also be employed. Forexample, it is preferable to stack a tantalum nitride film and atungsten film. Alternatively, a titanium film and an aluminum film maybe stacked. Alternatively, a two-layer structure where an aluminum filmis stacked over a tungsten film, a two-layer structure where a copperfilm is stacked over a copper-magnesium-aluminum alloy film, a two-layerstructure where a copper film is stacked over a titanium film, or atwo-layer structure where a copper film is stacked over a tungsten filmmay be employed.

Other examples include a three-layer structure where a titanium film ora titanium nitride film is formed, an aluminum film or a copper film isstacked over the titanium film or the titanium nitride film, and atitanium film or a titanium nitride film is formed thereover; and athree-layer structure where a molybdenum film or a molybdenum nitridefilm is formed, an aluminum film or a copper film is stacked over themolybdenum film or the molybdenum nitride film, and a molybdenum film ora molybdenum nitride film is formed thereover. Note that a transparentconductive material containing indium oxide, tin oxide, or zinc oxidemay be used.

A barrier layer may be provided over the conductor 542. The barrierlayer is preferably formed using a substance having a barrier propertyagainst oxygen or hydrogen. This structure can inhibit oxidation of theconductor 542 at the time of depositing the insulator 574.

A metal oxide can be used for the barrier layer, for example. Inparticular, an insulating film of aluminum oxide, hafnium oxide, galliumoxide, or the like, which has a barrier property against oxygen andhydrogen, is preferably used. Alternatively, silicon nitride formed by aCVD method may be used.

With the barrier layer, the range of choices for the material of theconductor 542 can be expanded. For example, a material having a lowoxidation resistance and high conductivity, such as tungsten oraluminum, can be used for the conductor 542. Moreover, for example, aconductor that can be easily deposited or processed can be used.

The insulator 550 functions as a first gate insulating film. Theinsulator 550 is preferably provided in the opening portion provided inthe insulator 580 with the oxide 530 c and the insulator 574therebetween.

With miniaturization and high integration of transistors, a problem suchas leakage current may arise because of a thinner gate insulating film.In that case, the insulator 550 may have a stacked-layer structure likethe second gate insulating film. When the insulator functioning as thegate insulating film has a stacked-layer structure of a high-k materialand a thermally stable material, a gate potential at the time when thetransistor operates can be reduced while the physical thickness ismaintained. Furthermore, the stacked-layer structure can be thermallystable and have a high dielectric constant.

The conductor 560 functioning as the first gate electrode includes theconductor 560 a and the conductor 560 b over the conductor 560 a. Likethe conductor 505 a, the conductor 560 a is preferably formed using aconductive material having a function of inhibiting diffusion ofimpurities such as a hydrogen atom, a hydrogen molecule, a watermolecule, and a copper atom. Alternatively, it is preferable to use aconductive material having a function of inhibiting diffusion of oxygen(e.g., at least one of an oxygen atom, an oxygen molecule, and thelike).

When the conductor 560 a has a function of inhibiting oxygen diffusion,the range of choices for the material of the conductor 560 b can beexpanded. That is, the conductor 560 a inhibits oxidation of theconductor 560 b, thereby preventing a decrease in conductivity.

As a conductive material having a function of inhibiting diffusion ofoxygen, tantalum, tantalum nitride, ruthenium, or ruthenium oxide ispreferably used, for example. For the conductor 560 a, the oxidesemiconductor that can be used as the oxide 530 can be used. In thatcase, when the conductor 560 b is deposited by a sputtering method, theconductor 560 a can have a reduced electric resistance value to be aconductor. This can be referred to as an OC (Oxide Conductor) electrode.

The conductor 560 b is preferably formed using a conductive materialcontaining tungsten, copper, or aluminum as its main component. Inaddition, since the conductor 560 functions as a wiring, a conductorhaving high conductivity is preferably used as the conductor 560 b. Forexample, a conductive material containing tungsten, copper, or aluminumas its main component can be used. The conductor 560 b may have astacked-layer structure, for example, a stack of any of the aboveconductive materials and titanium or titanium nitride.

The insulator 574 is positioned between the insulator 580 and thetransistor 510A. For the insulator 574, an insulating material having afunction of inhibiting diffusion of oxygen and impurities such as wateror hydrogen is preferably used. For example, aluminum oxide or hafniumoxide is preferably used. Moreover, it is possible to use, for example,a metal oxide such as magnesium oxide, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, ortantalum oxide or silicon nitride oxide, silicon nitride, or the like.

The insulator 574 can inhibit diffusion of impurities such as water andhydrogen contained in the insulator 580 into the oxide 530 b through theoxide 530 c and the insulator 550. Furthermore, oxidation of theconductor 560 due to excess oxygen contained in the insulator 580 can beinhibited.

The insulator 580, the insulator 582, and the insulator 584 eachfunction as an interlayer film.

Like the insulator 514, the insulator 582 preferably functions as abarrier insulating film that inhibits entry of impurities such as wateror hydrogen into the transistor 510A from the outside.

Like the insulator 516, the insulator 580 and the insulator 584preferably have a lower permittivity than the insulator 582. When amaterial with a low permittivity is used for the interlayer film, theparasitic capacitance generated between wirings can be reduced.

The transistor 510A may be electrically connected to another componentthrough a plug or a wiring such as the conductor 546 embedded in theinsulator 580, the insulator 582, and the insulator 584.

As a material for the conductor 546, a single layer or stacked layers ofa conductive material such as a metal material, an alloy material, ametal nitride material, or a metal oxide material can be used, as in thecase of the conductor 505. For example, it is preferable to use ahigh-melting-point material that has both heat resistance andconductivity, such as tungsten or molybdenum. Alternatively, it ispreferable to use a low-resistance conductive material such as aluminumor copper. The use of a low-resistance conductive material can reducewiring resistance.

For example, when the conductor 546 has a stacked-layer structure oftantalum nitride or the like, which is a conductor having a barrierproperty against hydrogen and oxygen, and tungsten, which has highconductivity, diffusion of impurities from the outside can be inhibitedwhile the conductivity of the wiring is maintained.

With the above structure, an OS transistor having a high on-statecurrent can be provided. Alternatively, an OS transistor having a lowoff-state current can be provided. Alternatively, in a semiconductordevice including an OS transistor, variations in electricalcharacteristics can be inhibited and the reliability can be improved.

<Structure Example 2 of Transistor >

A structure example of a transistor 510B is described with reference toFIG. 8A, FIG. 8B, and FIG. 8C. FIG. 8A is a top view of the transistor510B. FIG. 8B is a cross-sectional view of a portion indicated by adashed-dotted line L1-L2 in FIG. 8A. FIG. 8C is a cross-sectional viewof a portion indicated by a dashed-dotted line W1-W2 in FIG. 8A. Notethat for clarity of the drawing, some components are not illustrated inthe top view of FIG. 8A.

The transistor 510B is a modification example of the transistor 510A.Therefore, differences from the transistor 510A will be mainly describedto avoid repeated description.

The transistor 510B includes a region where the oxide 530 c, theinsulator 550, and the conductor 560 overlap with the conductor 542 (theconductor 542 a and the conductor 542 b). With this structure, atransistor having a high on-state current can be provided. Moreover, atransistor having high controllability can be provided.

The conductor 560 functioning as the first gate electrode includes theconductor 560 a and the conductor 560 b over the conductor 560 a. Likethe conductor 505 a, the conductor 560 a is preferably formed using aconductive material having a function of inhibiting diffusion ofimpurities such as a hydrogen atom, a hydrogen molecule, a watermolecule, and a copper atom. Alternatively, it is preferable to use aconductive material having a function of inhibiting diffusion of oxygen(e.g., at least one of an oxygen atom, an oxygen molecule, and thelike).

When the conductor 560 a has a function of inhibiting oxygen diffusion,the range of choices for the material of the conductor 560 b can beexpanded. That is, the conductor 560 a inhibits oxidation of theconductor 560 b, thereby preventing a decrease in conductivity.

The insulator 574 is preferably provided to cover the top surface and aside surface of the conductor 560, a side surface of the insulator 550,and a side surface of the oxide 530 c. For the insulator 574, aninsulating material having a function of inhibiting diffusion of oxygenand impurities such as water or hydrogen is preferably used. Forexample, aluminum oxide or hafnium oxide is preferably used. Moreover,it is possible to use, for example, a metal oxide such as magnesiumoxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, or tantalum oxide or silicon nitrideoxide, silicon nitride, or the like.

The insulator 574 can inhibit oxidation of the conductor 560. Moreover,the insulator 574 can inhibit diffusion of impurities such as water andhydrogen contained in the insulator 580 into the transistor 510B.

An insulator 576 (an insulator 576 a and an insulator 576 b) having abarrier property may be provided between the conductor 546 and theinsulator 580. Providing the insulator 576 can inhibit oxygen in theinsulator 580 from reacting with the conductor 546 and oxidizing theconductor 546.

Furthermore, with the insulator 576 having a barrier property, the rangeof choices for the material of the conductor used as the plug or thewiring can be expanded. The use of a metal material having an oxygenabsorbing property and high conductivity for the conductor 546, forexample, can provide a semiconductor device with low power consumption.Specifically, a material having a low oxidation resistance and highconductivity, such as tungsten or aluminum, can be used. Moreover, forexample, a conductor that can be easily deposited or processed can beused.

<Structure Example 3 of Transistor>

A structure example of a transistor 510C is described with reference toFIG. 9A, FIG. 9B, and FIG. 9C. FIG. 9A is a top view of the transistor510C. FIG. 9B is a cross-sectional view of a portion indicated by adashed-dotted line L1-L2 in FIG. 9A. FIG. 9C is a cross-sectional viewof a portion indicated by a dashed-dotted line W1-W2 in FIG. 9A. Notethat for clarity of the drawing, some components are not illustrated inthe top view of FIG. 9A.

The transistor 510C is a modification example of the transistor 510A.Therefore, differences from the transistor 510A will be mainly describedto avoid repeated description.

In the transistor 510C shown in FIG. 9A, FIG. 9B, and FIG. 9C, aconductor 547a is positioned between the conductor 542 a and the oxide530 b, and a conductor 547b is positioned between the conductor 542 band the oxide 530 b. Here, the conductor 542 a (the conductor 542 b) hasa region that extends beyond the top surface of the conductor 547 a (theconductor 547 b) and its side surface on the conductor 560 side and isin contact with the top surface of the oxide 530 b. For the conductors547, a conductor that can be used as the conductor 542 is used. It ispreferable that the thickness of the conductor 547 be at least greaterthan that of the conductor 542.

In the transistor 510C shown in FIG. 9A, FIG. 9B, and FIG. 9C, becauseof the above structure, the conductor 542 can be closer to the conductor560 than in the transistor 510A. Alternatively, the conductor 560 canoverlap with an end portion of the conductor 542 a and an end portion ofthe conductor 542 b. Thus, the effective channel length of thetransistor 510C can be shortened, and the on-state current and thefrequency characteristics can be improved.

The conductor 547 a (the conductor 547 b) is preferably provided tooverlap with the conductor 542 a (the conductor 542 b). With such astructure, the conductor 547 a (the conductor 547 b) can function as astopper to prevent over-etching of the oxide 530 b in etching forforming the opening in which the conductor 546 a (the conductor 546 b)is to be embedded.

The transistor 510C shown in FIG. 9A, FIG. 9B, and FIG. 9C may have astructure in which an insulator 545 is positioned over and in contactwith the insulator 544. The insulator 544 preferably functions as abarrier insulating film that inhibits entry of impurities such as wateror hydrogen and excess oxygen into the transistor 510C from theinsulator 580 side. The insulator 545 can be formed using an insulatorthat can be used for the insulator 544. In addition, the insulator 544may be formed using a nitride insulator such as aluminum nitride,aluminum titanium nitride, titanium nitride, silicon nitride, or siliconnitride oxide, for example.

Unlike in the transistor 510A shown in FIG. 7A, FIG. 7B, and FIG. 7C, inthe transistor 510C shown in FIG. 9A, FIG. 9B, and FIG. 9C, theconductor 505 may have a single-layer structure. In this case, aninsulating film to be the insulator 516 is formed over the patternedconductor 505, and an upper portion of the insulating film is removed bya CMP method or the like until the top surface of the conductor 505 isexposed. Preferably, the planarity of the top surface of the conductor505 is made favorable. For example, the average surface roughness (Ra)of the top surface of the conductor 505 is less than or equal to 1 nm,preferably less than or equal to 0.5 nm, and further preferably lessthan or equal to 0.3 nm. This allows the improvement in planarity of aninsulating layer formed over the conductor 505 and the increase incrystallinity of the oxide 530 b and the oxide 530 c.

<Structure Example 4 of Transistor>

A structure example of a transistor 510D is described with reference toFIG. 10A, FIG. 10B, and FIG. 10C. FIG. 10A is a top view of thetransistor 510D. FIG. 10B is a cross-sectional view of a portionindicated by a dashed-dotted line L1-L2 in FIG. 10A. FIG. 10C is across-sectional view of a portion indicated by a dashed-dotted lineW1-W2 in FIG. 10A. Note that for clarity of the drawing, some componentsare not illustrated in the top view in FIG. 10A.

The transistor 510D is a modification example of the above transistors.Therefore, differences from the above transistors will be mainlydescribed to avoid repeated description.

In FIG. 10A, FIG. 10B, and FIG. 10C, the conductor 503 is not provided,and the conductor 505 that has a function of a second gate is made tofunction also as a wiring. In addition, the insulator 550 is providedover the oxide 530 c and a metal oxide 552 is provided over theinsulator 550. In addition, the conductor 560 is provided over the metaloxide 552, and an insulator 570 is provided over the conductor 560.Furthermore, an insulator 571 is provided over the insulator 570.

The metal oxide 552 preferably has a function of inhibiting diffusion ofoxygen. When the metal oxide 552 that inhibits oxygen diffusion isprovided between the insulator 550 and the conductor 560, diffusion ofoxygen into the conductor 560 is inhibited. That is, a reduction in theamount of oxygen supplied to the oxide 530 can be inhibited. Moreover,oxidation of the conductor 560 due to oxygen can be inhibited.

Note that the metal oxide 552 may have a function of part of the firstgate. For example, the oxide semiconductor that can be used for theoxide 530 can be used for the metal oxide 552. In this case, when theconductor 560 is deposited by a sputtering method, the electricalresistance value of the metal oxide 552 is lowered so that the metaloxide 552 can be a conductive layer. This can be referred to as an OC(Oxide Conductor) electrode.

The metal oxide 552 may have a function of part of a gate insulatingfilm. Thus, when silicon oxide, silicon oxynitride, or the like is usedfor the insulator 550, a metal oxide that is a high-k material with ahigh dielectric constant is preferably used for the metal oxide 552.Such a stacked-layer structure can be thermally stable and can have ahigh dielectric constant. Thus, a gate potential that is applied whenthe transistor operates can be lowered while the physical thickness ismaintained. In addition, the equivalent oxide thickness (EOT) of aninsulating layer functioning as the gate insulating film can be reduced.

Although the metal oxide 552 in the transistor 510D is shown as a singlelayer, the metal oxide 552 may have a stacked-layer structure of two ormore layers. For example, a metal oxide functioning as part of the gateelectrode and a metal oxide functioning as part of the gate insulatingfilm may be stacked.

With the metal oxide 552 functioning as a gate electrode, the on-statecurrent of the transistor 510D can be increased without a reduction inthe influence of the electric field from the conductor 560. With themetal oxide 552 functioning as the gate insulating film, the distancebetween the conductor 560 and the oxide 530 is kept by the physicalthicknesses of the insulator 550 and the metal oxide 552, so thatleakage current between the conductor 560 and the oxide 530 can bereduced. Thus, with the stacked-layer structure of the insulator 550 andthe metal oxide 552, the physical distance between the conductor 560 andthe oxide 530 and the intensity of electric field applied from theconductor 560 to the oxide 530 can be easily adjusted as appropriate.

Specifically, the oxide semiconductor that can be used for the oxide 530can also be used for the metal oxide 552 when the resistance thereof isreduced. Alternatively, a metal oxide containing one kind or two or morekinds selected from hafnium, aluminum, gallium, yttrium, zirconium,tungsten, titanium, tantalum, nickel, germanium, magnesium, and the likecan be used.

It is particularly preferable to use an insulating layer containing anoxide of one or both of aluminum and hafnium, for example, aluminumoxide, hafnium oxide, or an oxide containing aluminum and hafnium(hafnium aluminate). In particular, hafnium aluminate has higher heatresistance than a hafnium oxide film. Therefore, hafnium aluminate ispreferable because it is less likely to be crystallized by heattreatment in a later step. Note that the metal oxide 552 is not anessential component. Design is appropriately set in consideration ofrequired transistor characteristics.

For the insulator 570, an insulating material having a function ofinhibiting the passage of oxygen and impurities such as water andhydrogen is preferably used. For example, aluminum oxide or hafniumoxide is preferably used. Thus, oxidation of the conductor 560 due tooxygen from above the insulator 570 can be inhibited. Moreover, entry ofimpurities such as water or hydrogen from above the insulator 570 intothe oxide 530 through the conductor 560 and the insulator 550 can beinhibited.

The insulator 571 functions as a hard mask. By providing the insulator571, the conductor 560 can be processed to have a side surface that issubstantially vertical; specifically, an angle formed by the sidesurface of the conductor 560 and a substrate surface can be greater thanor equal to 75° and less than or equal to 100°, preferably greater thanor equal to 80° and less than or equal to 95°.

Note that an insulating material having a function of inhibiting thepassage of oxygen and impurities such as water or hydrogen may be usedfor the insulator 571 so that the insulator 571 also functions as abarrier layer. In that case, the insulator 570 does not have to beprovided.

Parts of the insulator 570, the conductor 560, the metal oxide 552, theinsulator 550, and the oxide 530 c are selected and removed using theinsulator 571 as a hard mask, whereby their side surfaces can besubstantially aligned with each other and a surface of the oxide 530 bcan be partly exposed.

The transistor 510D includes a region 531 a and a region 531 b on a partof the exposed surface of the oxide 530 b. One of the region 531 a andthe region 531 b functions as a source region, and the other functionsas a drain region.

The region 531 a and the region 531 b can be formed by addition of animpurity element such as phosphorus or boron to the exposed surface ofthe oxide 530 b by an ion implantation method, an ion doping method, aplasma immersion ion implantation method, or plasma treatment, forexample. In this embodiment and the like, an “impurity element” refersto an element other than main constituent elements.

Alternatively, the region 531 a and the region 531 b can be formed insuch a manner that, after a part of the surface of the oxide 530 b isexposed, a metal film is formed and then heat treatment is performed sothat the element contained in the metal film is diffused into the oxide530 b.

The electrical resistivity of regions of the oxide 530 b to which theimpurity element has been added decreases. For that reason, the region531 a and the region 531 b are sometimes referred to as “impurityregions” or “low-resistance regions”.

The region 531 a and the region 531 b can be formed in a self-alignedmanner by using the insulator 571 and/or the conductor 560 as a mask.Thus, the conductor 560 does not overlap with the region 531 a and/orthe region 531 b, so that the parasitic capacitance can be reduced.Moreover, an offset region is not formed between a channel-formationregion and the source/drain region (the region 531 a or the region 531b). The formation of the region 531 a and the region 531 b in aself-aligned manner achieves an increase in on-state current, areduction in threshold voltage, and an improvement in operatingfrequency, for example.

Note that an offset region may be provided between the channel-formationregion and the source/drain region in order to further reduce theoff-state current. The offset region is a region where the electricalresistivity is high and the impurity element is not added. The offsetregion can be formed by the addition of the impurity element after theformation of an insulator 575. In this case, the insulator 575 serves asa mask like the insulator 571 or the like. Thus, the impurity element isnot added to a region of the oxide 530 b overlapping with the insulator575, so that the electrical resistivity of the region can be kept high.

The transistor 510D includes the insulator 575 on the side surfaces ofthe insulator 570, the conductor 560, the metal oxide 552, the insulator550, and the oxide 530 c. The insulator 575 is preferably an insulatorhaving a low dielectric constant. For example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, silicon oxide towhich fluorine is added, silicon oxide to which carbon is added, siliconoxide to which carbon and nitrogen are added, porous silicon oxide, aresin, or the like is preferably used. In particular, silicon oxide,silicon oxynitride, silicon nitride oxide, or porous silicon oxide ispreferably used for the insulator 575, in which case an excess-oxygenregion can be easily formed in the insulator 575 in a later step.Silicon oxide and silicon oxynitride are preferable because of theirthermal stability. The insulator 575 preferably has a function ofdiffusing oxygen.

The transistor 510D also includes the insulator 574 over the insulator575 and the oxide 530. The insulator 574 is preferably deposited by asputtering method. When a sputtering method is used, an insulatorcontaining few impurities such as water or hydrogen can be deposited.For example, aluminum oxide is preferably used for the insulator 574.

Note that an oxide film obtained by a sputtering method may extracthydrogen from the component over which the oxide film is deposited.Thus, the hydrogen concentration in the oxide 530 and the insulator 575can be reduced when the insulator 574 absorbs hydrogen and water fromthe oxide 530 and the insulator 575.

<Structure Example 5 of Transistor>

A structure example of a transistor 510E is described with reference toFIG. 11A, FIG. 11B, and FIG. 11C. FIG. 11A is a top view of thetransistor 510E. FIG. 11B is a cross-sectional view of a portionindicated by the dashed-dotted line L1-L2 in FIG. 11A. FIG. 11C is across-sectional view of a portion indicated by the dashed-dotted lineW1-W2 in FIG. 11A. Note that for clarity of the drawing, some componentsare not illustrated in the top view in FIG. 11A.

The transistor 510E is a modification example of the above transistors.Therefore, differences from the above transistors will be mainlydescribed to avoid repeated description.

In FIG. 11A, FIG. 11B, and FIG. 11C, the conductor 542 is not provided,and a part of the exposed surface of the oxide 530 b includes the region531 a and the region 531 b. One of the region 531 a and the region 531 bfunctions as a source region, and the other functions as a drain region.Moreover, an insulator 573 is included between the oxide 530 b and theinsulator 574.

The regions 531 (the region 531 a and the region 531 b) shown in FIG.11B are regions where an element described below is added to the oxide530 b. The regions 531 can be formed using a dummy gate, for example.

Specifically, a dummy gate is provided over the oxide 530 b, and theelement that reduces the resistance of the oxide 530 b is added usingthe dummy gate as a mask. That is, the element is added to regions ofthe oxide 530 that do not overlap with the dummy gate, whereby theregions 531 are formed. As a method of adding the element, an ionimplantation method by which an ionized source gas is subjected to massseparation and then added, an ion doping method by which an ionizedsource gas is added without mass separation, a plasma immersion ionimplantation method, or the like can be used.

Typical examples of the element that reduces the resistance of the oxide530 are boron and phosphorus. Moreover, hydrogen, carbon, nitrogen,fluorine, sulfur, chlorine, titanium, a rare gas, or the like may beused. Typical examples of the rare gas include helium, neon, argon,krypton, and xenon. The concentration of the element is measured bysecondary ion mass spectrometry (SIMS) or the like.

In particular, boron and phosphorus are preferable because an apparatusused in a manufacturing line for low-temperature polysilicon can beused, for example. Since the existing facility can be used, capitalinvestment can be reduced.

Next, an insulating film to be the insulator 573 and an insulating filmto be the insulator 574 may be formed over the oxide 530 b and the dummygate. Stacking the insulating film to be the insulator 573 and theinsulating film to be the insulator 574 can provide a region where theoxide 530 c and the insulator 550 overlap with the region 531.

Specifically, after an insulating film to be the insulator 580 isprovided over the insulating film to be the insulator 574, theinsulating film to be the insulator 580 is subjected to CMP (ChemicalMechanical Polishing) treatment, whereby a part of the insulating filmto be the insulator 580 is removed and the dummy gate is exposed. Then,when the dummy gate is removed, a part of the insulator 573 in contactwith the dummy gate is preferably also removed. Thus, the insulator 574and the insulator 573 are exposed at a side surface of an openingportion provided in the insulator 580, and the region 531 provided inthe oxide 530 b is partly exposed at the bottom surface of the openingportion. Next, an oxide film to be the oxide 530 c, an insulating filmto be the insulator 550, and a conductive film to be the conductor 560are formed in this order in the opening portion, and then the oxide filmto be the oxide 530 c, the insulating film to be the insulator 550, andthe conductive film to be the conductor 560 are partly removed by CMPtreatment or the like until the insulator 580 is exposed; thus, thetransistor shown in FIG. 11A, FIG. 11B, and FIG. 11C can be formed.

Note that the insulator 573 and the insulator 574 are not essentialcomponents. Design is appropriately set in consideration of requiredtransistor characteristics.

The cost of the transistor shown in FIG. 11A, FIG. 11B, and FIG. 11C canbe reduced because an existing apparatus can be used and the conductor542 is not provided.

<Structure Example 6 of Transistor>

A structure example of a transistor 510F is described with reference toFIG. 12A, FIG. 12B, and FIG. 12C. FIG. 12A is a top view of thetransistor 510F. FIG. 12B is a cross-sectional view of a portionindicated by a dashed-dotted line L1-L2 in FIG. 12A. FIG. 12C is across-sectional view of a portion indicated by a dashed-dotted lineW1-W2 in FIG. 12A. Note that for clarity of the drawing, some componentsare not illustrated in the top view in FIG. 12A.

The transistor 510F is a modification example of the transistor 510A.Therefore, differences from the above transistors will be mainlydescribed to avoid repeated description.

In the transistor 510A, a part of the insulator 574 is provided in theopening portion provided in the insulator 580 and covers the sidesurface of the conductor 560. Meanwhile, in the transistor 510F, anopening is formed by partly removing the insulator 580 and the insulator574.

An insulator 576 (an insulator 576 a and an insulator 576 b) having abarrier property may be provided between the conductor 546 and theinsulator 580. Providing the insulator 576 can inhibit oxygen in theinsulator 580 from reacting with the conductor 546 and oxidizing theconductor 546.

Note that when an oxide semiconductor is used as the oxide 530, theoxide 530 preferably has a stacked-layer structure of a plurality ofoxide layers that differ in the atomic ratio of metal atoms.Specifically, the atomic proportion of the element M in the constituentelements in the metal oxide used as the oxide 530 a is preferably higherthan the atomic proportion of the element M in the constituent elementsin the metal oxide used as the oxide 530 b. In addition, the atomicratio of the element M to In in the metal oxide used as the oxide 530 ais preferably higher than the atomic ratio of the element M to In in themetal oxide used as the oxide 530 b. Furthermore, the atomic ratio of Into the elementMin the metal oxide used as the oxide 530 b is preferablyhigher than the atomic ratio of In to the element M in the metal oxideused as the oxide 530 a. A metal oxide that can be used as the oxide 530a or the oxide 530 b can be used as the oxide 530 c.

The oxide 530 a, the oxide 530 b, and the oxide 530 c preferably havecrystallinity, and in particular, it is preferable to use a CAAC-OS. Anoxide having crystallinity, such as a CAAC-OS, has a dense structurewith small amounts of impurities and defects (e.g., oxygen vacancies)and high crystallinity. This can inhibit extraction of oxygen from theoxide 530 b by the source electrode or the drain electrode. This canreduce extraction of oxygen from the oxide 530 b even when heattreatment is performed; hence, the transistor 510F is stable againsthigh temperatures (i.e., thermal budget) in the manufacturing process.

Note that one or both of the oxide 530 a and the oxide 530 c may beomitted. The oxide 530 may be a single layer of the oxide 530 b. In thecase where the oxide 530 is a stack of the oxide 530 a, the oxide 530 b,and the oxide 530 c, the energy of the conduction band minimum of eachof the oxide 530 a and the oxide 530 c is preferably higher than theenergy of the conduction band minimum of the oxide 530 b. In otherwords, the electron affinity of each of the oxide 530 a and the oxide530 c is preferably smaller than the electron affinity of the oxide 530b. In that case, for the oxide 530 c, a metal oxide that can be used forthe oxide 530 a is preferably used. Specifically, the atomic proportionof the element M in the constituent elements in the metal oxide used asthe oxide 530 c is preferably higher than the atomic proportion of theelement M in the constituent elements in the metal oxide used as theoxide 530 b. Moreover, the atomic ratio of the element M to In in themetal oxide used as the oxide 530 c is preferably higher than the atomicratio of the element M to In in the metal oxide used as the oxide 530 b.Furthermore, the atomic ratio of In to the elementMin the metal oxideused as the oxide 530 b is preferably higher than the atomic ratio of Into the element M in the metal oxide used as the oxide 530 c.

The energy level of the conduction band minimum gradually changes atjunction portions of the oxide 530 a, the oxide 530 b, and the oxide 530c. In other words, the energy level of the conduction band minimum atthe junction portions of the oxide 530 a, the oxide 530 b, and the oxide530 c continuously changes or is continuously connected. To obtain this,the density of defect states in a mixed layer formed at the interfacebetween the oxide 530 a and the oxide 530 b and the interface betweenthe oxide 530 b and the oxide 530 c is preferably made low.

Specifically, when the oxide 530 a and the oxide 530 b or the oxide 530b and the oxide 530 c contain a common element (as a main component) inaddition to oxygen, a mixed layer with a low density of defect statescan be formed. For example, in the case where the oxide 530 b is anIn—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or thelike may be used as the oxide 530 a and the oxide 530 c. In addition,the oxide 530 c may have a stacked-layer structure. For example, it ispossible to employ a stacked-layer structure of an In—Ga—Zn oxide and aGa—Zn oxide over the In—Ga—Zn oxide, or a stacked-layer structure of anIn—Ga—Zn oxide and gallium oxide over the In—Ga—Zn oxide. In otherwords, the oxide 530 c may employ a stacked-layer structure of anIn—Ga—Zn oxide and an oxide that does not contain In.

Specifically, as the oxide 530 a, a metal oxide with In:Ga:Zn=1:3:4[atomic ratio] or 1:1:0.5 [atomic ratio] is used. As the oxide 530 b, ametal oxide with In:Ga:Zn=4:2:3 [atomic ratio] or 3:1:2 [atomic ratio]is used. As the oxide 530 c, a metal oxide with In:Ga:Zn=1:3:4 [atomicratio], In:Ga:Zn=4:2:3 [atomic ratio], Ga:Zn=2:1 [atomic ratio], orGa:Zn=2:5 [atomic ratio] is used. Furthermore, specific examples of thecase where the oxide 530 c has a stacked-layer structure include astacked-layer structure of In:Ga:Zn=4:2:3 [atomic ratio] and Ga:Zn=2:1[atomic ratio], a stacked-layer structure of In:Ga:Zn=4:2:3 [atomicratio] and Ga:Zn=2:5 [atomic ratio], and a stacked-layer structure ofIn:Ga:Zn=4:2:3 [atomic ratio] and gallium oxide.

At this time, the oxide 530 b serves as a main carrier path. When theoxide 530 a and the oxide 530 c have the above structure, the density ofdefect states at the interface between the oxide 530 a and the oxide 530b and the interface between the oxide 530 b and the oxide 530 c can bemade low. Thus, the influence of interface scattering on carrierconduction is small, and the transistor 510F can have a high on-statecurrent and high frequency characteristics. Note that in the case wherethe oxide 530 c has a stacked-layer structure, in addition to the effectof reducing the density of defect states at the interface between theoxide 530 b and the oxide 530 c, the effect of inhibiting diffusion ofthe constituent element of the oxide 530 c to the insulator 550 side isexpected. More specifically, the oxide 530 c has a stacked-layerstructure and the oxide that does not contain In is positioned at theupper part of the stacked-layer structure, whereby the amount of In thatwould diffuse to the insulator 550 side can be reduced. Since theinsulator 550 functions as a gate insulator, the transistor has defectsin characteristics when In diffuses. Thus, when the oxide 530 c has astacked-layer structure, a highly reliable display device can beprovided.

A metal oxide functioning as an oxide semiconductor is preferably usedas the oxide 530. For example, as the metal oxide to be thechannel-formation region in the oxide 530, a metal oxide having abandgap of 2 eV or larger, preferably 2.5 eV or larger is preferablyused. With the use of a metal oxide having such a wide bandgap, theoff-state current of the transistor can be reduced. With such atransistor, a semiconductor device with low power consumption can beprovided.

<Structure Example 7 of Transistor>

A structure example of a transistor 510G is described with reference toFIG. 13A and FIG. 13B. The transistor 510G is a modification example ofthe transistor 500. Therefore, differences from the above transistorswill be mainly described to avoid repeated description. Note that thestructure shown in FIG. 13A and FIG. 13B can be employed for othertransistors, such as the transistor 300, included in the semiconductordevice of one embodiment of the present invention.

FIG. 13A is a cross-sectional view of the transistor 510G in the channellength direction, and FIG. 13B is a cross-sectional view of thetransistor 510G in the channel width direction. The transistor 510Gshown in FIG. 13A and FIG. 13B is different from the transistor 500shown in FIG. 6A and FIG. 6B in including the insulator 402 and theinsulator 404. Another difference from the transistor 500 shown in FIG.6A and FIG. 6B is that the insulator 551 is provided in contact with aside surface of the conductor 540 a and the insulator 551 is provided incontact with a side surface of the conductor 540 b. Another differencefrom the transistor 500 shown in FIG. 6A and FIG. 6B is that theinsulator 520 is not provided.

In the transistor 510G shown in FIG. 13A and FIG. 13B, the insulator 402is provided over the insulator 512. In addition, the insulator 404 isprovided over the insulator 574 and the insulator 402.

The transistor 510G shown in FIG. 13A and FIG. 13B has a structure inwhich the insulator 514, the insulator 516, the insulator 522, theinsulator 524, the insulator 544, the insulator 580, and the insulator574 are patterned and covered with the insulator 404. That is, theinsulator 404 is in contact with the top surface of the insulator 574, aside surface of the insulator 574, a side surface of the insulator 580,a side surface of the insulator 544, a side surface of the insulator524, a side surface of the insulator 522, a side surface of theinsulator 516, a side surface of the insulator 514, and the top surfaceof the insulator 402. Thus, the oxide 530 and the like are isolated fromthe outside by the insulator 404 and the insulator 402.

It is preferable that the insulator 402 and the insulator 404 havehigher capability of inhibiting diffusion of hydrogen (e.g., at leastone of a hydrogen atom, a hydrogen molecule, and the like) or a watermolecule. For example, the insulator 402 and the insulator 404 arepreferably formed using silicon nitride or silicon nitride oxide with ahigh hydrogen barrier property. This can inhibit diffusion of hydrogenor the like into the oxide 530, thereby inhibiting the degradation ofthe characteristics of the transistor 510G. Consequently, thereliability of the semiconductor device including an OS transistor canbe increased.

The insulator 551 is provided in contact with the insulator 581, theinsulator 404, the insulator 574, the insulator 580, and the insulator544. The insulator 551 preferably has a function of inhibiting diffusionof hydrogen or water molecules. For example, as the insulator 551, aninsulator such as silicon nitride, aluminum oxide, or silicon nitrideoxide that has a high hydrogen barrier property is preferably used. Inparticular, silicon nitride is suitably used for the insulator 551because of its high hydrogen barrier property. The use of a materialhaving a high hydrogen barrier property for the insulator 551 caninhibit diffusion of impurities such as water or hydrogen from theinsulator 580 and the like into the oxide 530 through the conductor 540a and the conductor 540 b. Furthermore, oxygen contained in theinsulator 580 can be inhibited from being absorbed by the conductor 540a and the conductor 540 b. Consequently, the reliability of thesemiconductor device including an OS transistor can be increased.

FIG. 14 is a cross-sectional view showing a structure example of thesemiconductor device in the case where the transistor 500 and thetransistor 300 have the structure shown in FIG. 13A and FIG. 13B. Theinsulator 551 is provided on the side surface of the conductor 546.

FIG. 15A and FIG. 15B show a modification example of the transistorshown in FIG. 13A and FIG. 13B. FIG. 15A is a cross-sectional view ofthe transistor in the channel length direction, and FIG. 15B is across-sectional view of the transistor in the channel width direction.The transistor shown in FIG. 15A and FIG. 15B is different from thetransistor shown in FIG. 13A and FIG. 13B in that the oxide 530 c has atwo-layer structure of an oxide 530 c 1 and an oxide 530 c 2.

The oxide 530 c 1 is in contact with the top surface of the insulator524, a side surface of the oxide 530 a, the top surface and a sidesurface of the oxide 530 b, side surfaces of the conductor 542 a and theconductor 542 b, a side surface of the insulator 544, and a side surfaceof the insulator 580. The oxide 530 c 2 is in contact with the insulator550.

An In—Zn oxide can be used as the oxide 530 c 1, for example. For theoxide 530 c 2, it is possible to use a material similar to the materialthat can be used for the oxide 530 c when the oxide 530 c has asingle-layer structure. For example, as the oxide 530 c 2, a metal oxidewith In:Ga:Zn=1:3:4 [atomic ratio], Ga:Zn=2:1 [atomic ratio], or Ga:Zn=2:5 [atomic ratio] can be used.

When the oxide 530 c has a two-layer structure of the oxide 530 c 1 andthe oxide 530 c 2, the on-state current of the transistor can beincreased as compared with the case where the oxide 530 c has asingle-layer structure. Thus, a transistor can be a power MOStransistor, for example. Note that the oxide 530 c included in thetransistor shown in FIG. 6A and FIG. 6B can also have a two-layerstructure of the oxide 530 c 1 and the oxide 530 c 2.

The transistor shown in FIG. 15A and FIG. 15B can be employed for thetransistor 500, the transistor 300, or both thereof.

Note that this embodiment can be implemented in appropriate combinationwith the other embodiments described in this specification.

Embodiment 3

In this embodiment, an oxide semiconductor that is a kind of metal oxidewill be described.

The metal oxide preferably contains at least indium or zinc. Inparticular, indium and zinc are preferably contained. In addition, oneor more kinds selected from aluminum, gallium, yttrium, tin, and thelike is preferably contained. Furthermore, one or more kinds selectedfrom boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, cobalt, and the like may be contained.

<Classification of Crystal Structure>

First, the classification of the crystal structures of an oxidesemiconductor will be described with reference to FIG. 16A. FIG. 16A isa diagram showing the classification of crystal structures of an oxidesemiconductor, typically IGZO (a metal oxide containing In, Ga, and Zn).

As shown in FIG. 16A, an oxide semiconductor is roughly classified into“Amorphous”, “Crystalline”, and “Crystal”. The term “Amorphous” includescompletely amorphous. The term “Crystalline” includes CAAC(c-axis-aligned crystalline), nc (nanocrystalline), and CAC(cloud-aligned composite). Note that the term “Crystalline” excludessingle crystal, poly crystal, and completely amorphous. The term“Crystal” includes single crystal and poly crystal.

Note that the structures in the thick frame in FIG. 16A are in anintermediate state between “Amorphous” and “Crystal”, and belong to anew crystalline phase. That is, these structures are completelydifferent from “Amorphous”, which is energetically unstable, and“Crystal”.

A crystal structure of a film or a substrate can be evaluated with anX-ray diffraction (XRD) spectrum. FIG. 16B shows an XRD spectrum, whichis obtained by GIXD (Grazing-Incidence XRD) measurement, of a CAAC-IGZOfilm classified into “Crystalline”. Note that a GIXD method is alsoreferred to as a thin film method or a Seemann-Bohlin method. The XRDspectrum that is shown in FIG. 16B and obtained by GIXD measurement ishereinafter simply referred to as an XRD spectrum. The CAAC-IGZO filmshown in FIG. 16B has a composition in the vicinity of In:Ga:Zn=4:2:3[atomic ratio]. The CAAC-IGZO film shown in FIG. 16B has a thickness of500 nm.

As shown in FIG. 16B, a clear peak indicating crystallinity is detectedin the XRD spectrum of the CAAC-IGZO film. Specifically, a peakindicating c-axis alignment is detected at 2θ of around 31° in the XRDspectrum of the CAAC-IGZO film. As shown in FIG. 16B, the peak at 2θ ofaround 31° is asymmetric with respect to the axis of the angle at whichthe peak intensity is detected.

A crystal structure of a film or a substrate can also be evaluated witha diffraction pattern obtained by a nanobeam electron diffraction (NBED)method (such a pattern is also referred to as a nanobeam electrondiffraction pattern). FIG. 16C shows a diffraction pattern of theCAAC-IGZO film. FIG. 16C shows a diffraction pattern obtained with theNBED method in which an electron beam is incident in the directionparallel to the substrate. The composition of the CAAC-IGZO film shownin FIG. 16C is In:Ga:Zn=4:2:3 [atomic ratio] or the neighborhoodthereof. In the nanobeam electron diffraction method, electrondiffraction is performed with a probe diameter of 1 nm.

As shown in FIG. 16C, a plurality of spots indicating c-axis alignmentare observed in the diffraction pattern of the CAAC-IGZO film.

<<Structure of Oxide Semiconductor>>

Oxide semiconductors might be classified in a manner different from thatin FIG. 16A when classified in terms of the crystal structure. Oxidesemiconductors are classified into a single crystal oxide semiconductorand a non-single-crystal oxide semiconductor, for example. Examples ofthe non-single-crystal oxide semiconductor include the above-describedCAAC-OS and nc-OS. Other examples of the non-single-crystal oxidesemiconductor include a polycrystalline oxide semiconductor, anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

Here, the above-described CAAC-OS, nc-OS, and a-like OS will bedescribed in detail.

[CAAC-OS]

The CAAC-OS is an oxide semiconductor that has a plurality of crystalregions each of which has c-axis alignment in a particular direction.Note that the particular direction refers to the thickness direction ofa CAAC-OS film, the normal direction of the surface where the CAAC-OSfilm is formed, or the normal direction of the surface of the CAAC-OSfilm. The crystal region refers to a region having a periodic atomicarrangement. When an atomic arrangement is regarded as a latticearrangement, the crystal region also refers to a region with a uniformlattice arrangement. The CAAC-OS has a region where a plurality ofcrystal regions are connected in the a-b plane direction, and the regionhas distortion in some cases. Note that the distortion refers to aportion where the direction of a lattice arrangement changes between aregion with a uniform lattice arrangement and another region with auniform lattice arrangement in a region where a plurality of crystalregions are connected. That is, the CAAC-OS is an oxide semiconductorhaving c-axis alignment and having no clear alignment in the a-b planedirection.

Note that each of the plurality of crystal regions is formed of one ormore fine crystals (crystals each of which has a maximum diameter ofless than 10 nm). In the case where the crystal region is formed of onefine crystal, the maximum diameter of the crystal region is less than 10nm. In the case where the crystal region is formed of a large number offine crystals, the size of the crystal region may be approximatelyseveral tens of nanometers.

In the case of an In—M—Zn oxide (the element M is one or more kindsselected from aluminum, gallium, yttrium, tin, titanium, and the like),the CAAC-OS tends to have a layered crystal structure (also referred toas a layered structure) in which a layer containing indium (In) andoxygen (hereinafter, an In layer) and a layer containing the element M,zinc (Zn), and oxygen (hereinafter, an (M, Zn) layer) are stacked.Indium and the element M can be replaced with each other. Therefore,indium may be contained in the (M, Zn) layer. In addition, the element Mmay be contained in the In layer. Note that Zn may be contained in theIn layer. Such a layered structure is observed as a lattice image in ahigh-resolution TEM image, for example.

When the CAAC-OS film is subjected to structural analysis byOut-of-plane XRD measurement with an XRD apparatus using θ/2θ scanning,for example, a peak indicating c-axis alignment is detected at 2θ of 31°or around 31°. Note that the position of the peak indicating c-axisalignment (the value of 2θ) may change depending on the kind,composition, or the like of the metal element contained in the CAAC-OS.

For example, a plurality of bright spots are observed in the electrondiffraction pattern of the CAAC-OS film. Note that one spot and anotherspot are observed point-symmetrically with a spot of the incidentelectron beam passing through a sample (also referred to as a directspot) as the symmetric center.

When the crystal region is observed from the particular direction, alattice arrangement in the crystal region is basically a hexagonallattice arrangement; however, a unit lattice is not always a regularhexagon and is a non-regular hexagon in some cases. A pentagonal latticearrangement, a heptagonal lattice arrangement, and the like are includedin the distortion in some cases. Note that a clear crystal grainboundary (grain boundary) cannot be observed even in the vicinity of thedistortion in the CAAC-OS. That is, formation of a crystal grainboundary is inhibited by the distortion of lattice arrangement. This isprobably because the CAAC-OS can tolerate distortion owing to a lowdensity of arrangement of oxygen atoms in the a-b plane direction, aninteratomic bond distance changed by substitution of a metal atom, andthe like.

A crystal structure in which a clear crystal grain boundary is observedis what is called polycrystal. It is highly probable that the crystalgrain boundary becomes a recombination center and captures carriers andthus decreases the on-state current and field-effect mobility of atransistor, for example. Thus, the CAAC-OS in which no clear crystalgrain boundary is observed is one of crystalline oxides having a crystalstructure suitable for a semiconductor layer of a transistor. Note thatZn is preferably contained to form the CAAC-OS. For example, an In—Znoxide and an In—Ga—Zn oxide are suitable because they can inhibitgeneration of a crystal grain boundary as compared with an In oxide.

The CAAC-OS is an oxide semiconductor with high crystallinity in whichno clear crystal grain boundary is observed. Thus, in the CAAC-OS, areduction in electron mobility due to the crystal grain boundary isunlikely to occur. Moreover, since the crystallinity of an oxidesemiconductor might be decreased by entry of impurities, formation ofdefects, or the like, the CAAC-OS can be regarded as an oxidesemiconductor that has small amounts of impurities and defects (e.g.,oxygen vacancies). Thus, an oxide semiconductor including the CAAC-OS isphysically stable. Therefore, the oxide semiconductor including theCAAC-OS is resistant to heat and has high reliability. In addition, theCAAC-OS is stable with respect to high temperature in the manufacturingprocess (i.e., thermal budget). Accordingly, the use of the CAAC-OS forthe OS transistor can extend the degree of freedom of the manufacturingprocess.

[nc-O S]

In the nc-OS, a microscopic region (e.g., a region with a size greaterthan or equal to 1 nm and less than or equal to 10 nm, in particular, aregion with a size greater than or equal to 1 nm and less than or equalto 3 nm) has a periodic atomic arrangement. In other words, the nc-OSincludes a fine crystal. Note that the size of the fine crystal is, forexample, greater than or equal to 1 nm and less than or equal to 10 nm,particularly greater than or equal to 1 nm and less than or equal to 3nm; thus, the fine crystal is also referred to as a nanocrystal.Furthermore, there is no regularity of crystal orientation betweendifferent nanocrystals in the nc-OS. Thus, the orientation in the wholefilm is not observed. Accordingly, the nc-OS cannot be distinguishedfrom an a-like OS or an amorphous oxide semiconductor by some analysismethods. For example, when an nc-OS film is subjected to structuralanalysis by Out-of-plane XRD measurement with an XRD apparatus usingθ/2θ scanning, a peak indicating crystallinity is not detected.Furthermore, a diffraction pattern like a halo pattern is observed whenthe nc-OS film is subjected to electron diffraction (also referred to asselected-area electron diffraction) using an electron beam with a probediameter larger than the diameter of a nanocrystal (e.g., larger than orequal to 50 nm). Meanwhile, in some cases, a plurality of spots in aring-like region with a direct spot as the center are observed in theobtained electron diffraction pattern when the nc-OS film is subjectedto electron diffraction (also referred to as nanobeam electrondiffraction) using an electron beam with a probe diameter nearly equalto or smaller than the diameter of a nanocrystal (e.g., 1 nm or largerand 30 nm or smaller).

[a-like OS]

The a-like OS is an oxide semiconductor having a structure between thoseof the nc-OS and the amorphous oxide semiconductor. The a-like OSincludes a void or a low-density region. That is, the a-like OS has lowcrystallinity as compared with the nc-OS and the CAAC-OS. Moreover, thea-like OS has higher hydrogen concentration in the film than the nc-OSand the CAAC-OS.

<<Structure of Oxide Semiconductor>>

Next, the above-described CAC-OS is described in detail. Note that theCAC-OS relates to the material composition.

[CAC-OS]

The CAC-OS refers to one composition of a material in which elementsconstituting a metal oxide are unevenly distributed with a size greaterthan or equal to 0.5 nm and less than or equal to 10 nm, preferablygreater than or equal to 1 nm and less than or equal to 3 nm, or asimilar size, for example. Note that a state in which one or more metalelements are unevenly distributed and regions including the metalelement(s) are mixed with a size greater than or equal to 0.5 nm andless than or equal to 10 nm, preferably greater than or equal to 1 nmand less than or equal to 3 nm, or a similar size in a metal oxide ishereinafter referred to as a mosaic pattern or a patch-like pattern.

In addition, the CAC-OS has a composition in which materials areseparated into a first region and a second region to form a mosaicpattern, and the first regions are distributed in the film (thiscomposition is hereinafter also referred to as a cloud-likecomposition). That is, the CAC-OS is a composite metal oxide having acomposition in which the first regions and the second regions are mixed.

Note that the atomic ratios of In, Ga, and Zn to the metal elementscontained in the CAC-OS in an In—Ga—Zn oxide are denoted with [In],[Ga], and [Zn], respectively. For example, the first region in theCAC-OS in the In—Ga—Zn oxide has [In] higher than [In] in thecomposition of the CAC-OS film. Moreover, the second region has [Ga]higher than [Ga] in the composition of the CAC-OS film. For example, thefirst region has higher [In] and lower [Ga] than the second region.Moreover, the second region has higher [Ga] and lower [In] than thefirst region.

Specifically, the first region includes indium oxide, indium zinc oxide,or the like as its main component. The second region includes galliumoxide, gallium zinc oxide, or the like as its main component. That is,the first region can be referred to as a region containing In as itsmain component. The second region can be referred to as a regioncontaining Ga as its main component.

Note that a clear boundary between the first region and the secondregion cannot be observed in some cases.

For example, in EDX mapping obtained by energy dispersive X-rayspectroscopy (EDX), it is confirmed that the CAC-OS in the In—Ga—Znoxide has a composition in which the region containing In as its maincomponent (the first region) and the region containing Ga as its maincomponent (the second region) are unevenly distributed and mixed.

In the case where the CAC-OS is used for a transistor, a switchingfunction (On/Off switching function) can be given to the CAC-OS owing tothe complementary action of the conductivity derived from the firstregion and the insulating property derived from the second region. ACAC-OS has a conducting function in part of the material and has aninsulating function in another part of the material; as a whole, theCAC-OS has a function of a semiconductor. Separation of the conductingfunction and the insulating function can maximize each function.Accordingly, when the CAC-OS is used for a transistor, high on-statecurrent (I_(on)), high field-effect mobility (μ), and excellentswitching operation can be achieved.

An oxide semiconductor has various structures with different properties.Two or more kinds among the amorphous oxide semiconductor, thepolycrystalline oxide semiconductor, the a-like OS, the CAC-OS, thenc-OS, and the CAAC-OS may be included in an oxide semiconductor of oneembodiment of the present invention.

<Transistor Including Oxide Semiconductor>

Next, the case where the above oxide semiconductor is used for atransistor is described.

When the above oxide semiconductor is used for a transistor, atransistor with high field-effect mobility can be achieved. In addition,a transistor having high reliability can be achieved.

An oxide semiconductor with a low carrier concentration is preferablyused for the transistor. For example, the carrier concentration of anoxide semiconductor is lower than or equal to 1×10¹⁷ cm⁻³, preferablylower than or equal to 1×10¹⁵ cm⁻³, further preferably lower than orequal to 1×10¹³ cm⁻³, still further preferably lower than or equal to1×10¹¹ cm⁻³, yet further preferably lower than 1×10¹⁰ cm⁻³, and higherthan or equal to 1×10⁻⁹ cm⁻³. In order to reduce the carrierconcentration of an oxide semiconductor film, the impurity concentrationin the oxide semiconductor film is reduced so that the density of defectstates can be reduced. In this specification and the like, a state witha low impurity concentration and a low density of defect states isreferred to as a highly purified intrinsic or substantially highlypurified intrinsic state. Note that an oxide semiconductor having a lowcarrier concentration may be referred to as a highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states and thus hasa low density of trap states in some cases.

Electric charge trapped by the trap states in the oxide semiconductortakes a long time to disappear and might behave like fixed electriccharge. Thus, a transistor whose channel formation region is formed inan oxide semiconductor with a high density of trap states has unstableelectrical characteristics in some cases.

Accordingly, in order to obtain stable electrical characteristics of atransistor, reducing the impurity concentration in an oxidesemiconductor is effective. In order to reduce the impurityconcentration in the oxide semiconductor, it is preferable that theimpurity concentration in an adjacent film be also reduced. Examples ofimpurities include hydrogen, nitrogen, an alkali metal, an alkalineearth metal, iron, nickel, and silicon.

<Impurity>

Here, the influence of each impurity in the oxide semiconductor isdescribed.

When silicon or carbon, which is one of Group 14 elements, is containedin the oxide semiconductor, defect states are formed in the oxidesemiconductor. Thus, the concentration of silicon or carbon in the oxidesemiconductor and the concentration of silicon or carbon in the vicinityof an interface with the oxide semiconductor (the concentration obtainedby secondary ion mass spectrometry (SIMS)) are each set lower than orequal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷atoms/cm³.

When the oxide semiconductor contains an alkali metal or an alkalineearth metal, defect states are formed and carriers are generated in somecases. Thus, a transistor using an oxide semiconductor that contains analkali metal or an alkaline earth metal is likely to have normally-oncharacteristics. Thus, the concentration of an alkali metal or analkaline earth metal in the oxide semiconductor, which is obtained bySIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower thanor equal to 2×10¹⁶ atoms/cm³.

Furthermore, when the oxide semiconductor contains nitrogen, the oxidesemiconductor easily becomes n-type by generation of electrons servingas carriers and an increase in carrier concentration. As a result, atransistor using as a semiconductor an oxide semiconductor containingnitrogen is likely to have normally-on characteristics. When nitrogen iscontained in the oxide semiconductor, a trap state is sometimes formed.This might make the electrical characteristics of the transistorunstable. Therefore, the concentration of nitrogen in the oxidesemiconductor, which is obtained by SIMS, is set lower than 5×10¹⁹atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁸ atoms/cm³, and still furtherpreferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in the oxide semiconductor reacts with oxygen bondedto a metal atom to be water, and thus forms an oxygen vacancy in somecases. Entry of hydrogen into the oxygen vacancy generates an electronserving as a carrier in some cases. Furthermore, bonding of part ofhydrogen to oxygen bonded to a metal atom causes generation of anelectron serving as a carrier in some cases. Thus, a transistor using anoxide semiconductor containing hydrogen is likely to have normally-oncharacteristics. Accordingly, hydrogen in the oxide semiconductor ispreferably reduced as much as possible. Specifically, the hydrogenconcentration in the oxide semiconductor, which is obtained by SIMS, isset lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³,further preferably lower than 5×10¹⁸ atoms/cm³, and still furtherpreferably lower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor with sufficiently reduced impurities is usedfor the channel formation region of the transistor, stable electricalcharacteristics can be given.

Note that the composition, structure, method, and the like described inthis embodiment can be used in appropriate combination with thecompositions, structures, methods, and the like described in the otherembodiments and the like.

REFERENCE NUMERALS

Q11: charge, Q12: charge, C11: capacitor, C12: capacitor, S1: signal,S2: signal, S3: signal, S4: signal, S5: signal, SW1: switch, SW1_1:switch, SW1_2: switch, SW2: switch, SW2_1: switch, SW2_2: switch, SW3:switch, SW3_1: switch, SW3_2: switch, INP: input terminal, INM: inputterminal, OUTP: output terminal, OUTM: output terminal, T11: terminal,T12: terminal, T13: terminal, T14: terminal, T15: terminal, T16:terminal, T21: terminal, T22: terminal, T23: terminal, T24: terminal,T31: terminal, T32: terminal, T33: terminal, T34: terminal, T_BN:terminal, T_BP: terminal, T_CN: terminal, T_COM: terminal, T_CP:terminal, T_VDD: terminal, VDD: power supply potential, 11: transistor,12: transistor, 13: transistor, 20: chopping circuit, 20_1: choppingcircuit, 20_2: chopping circuit, 21: transistor, 22: transistor, 23:transistor, 24: transistor, 30: amplifier, 31: transistor, 32:transistor, 33: transistor, 34: transistor, 35: transistor, 36:transistor, 37: transistor, 38: transistor, 39: transistor, 41:transistor, 42: transistor, 43: transistor, 44: transistor, 100:semiconductor device, 300: transistor, 311: substrate, 313:semiconductor region, 314 a: low-resistance region, 314 b:low-resistance region, 315: insulator, 316: conductor, 320: insulator,322: insulator, 324: insulator, 326: insulator, 328: conductor, 330:conductor, 350: insulator, 352: insulator, 354: insulator, 356:conductor, 360: insulator, 362: insulator, 364: insulator, 366:conductor, 370: insulator, 372: insulator, 374: insulator, 376:conductor, 380: insulator, 382: insulator, 384: insulator, 386:conductor, 402: insulator, 404: insulator, 500: transistor, 503:conductor, 503 a: conductor, 503 b: conductor, 505: conductor, 505 a:conductor, 505 b: conductor, 510: insulator, 510A: transistor, 510B:transistor, 510C: transistor, 510D: transistor, 510E: transistor, 510F:transistor, 510G: transistor, 511: insulator, 512: insulator, 514:insulator, 516: insulator, 518: conductor, 520: insulator, 521:insulator, 522: insulator, 524: insulator, 530: oxide, 530 a: oxide, 530b: oxide, 530 c: oxide, 530 c 1: oxide, 530 c 2: oxide, 531: region, 531a: region, 531 b: region, 540 a: conductor, 540 b: conductor, 542:conductor, 542 a: conductor, 542 b: conductor, 543: region, 543 a:region, 543 b: region, 544: insulator, 545: insulator, 546: conductor,546 a: conductor, 546 b: conductor, 547: conductor, 547 a: conductor,547 b: conductor, 548: conductor, 550: insulator, 551: insulator, 552:metal oxide, 560: conductor, 560 a: conductor, 560 b: conductor, 570:insulator, 571: insulator, 573: insulator, 574: insulator, 575:insulator, 576: insulator, 576 a: insulator, 576 b: insulator, 580:insulator, 581: insulator, 582: insulator, 584: insulator, 586:insulator, 600: capacitive element, 610: conductor, 612: conductor, 620:conductor, 630: insulator, 650: insulator

1. A semiconductor device comprising: a switch; first and secondcapacitors; first and second chopping circuits; an amplifier; first andsecond input terminals; and first and second output terminals, whereinthe amplifier comprises a non-inverting input terminal, an invertinginput terminal, an inverting output terminal, and a non-inverting outputterminal, wherein the semiconductor device electrically connects thefirst input terminal and one terminal of the first capacitor,electrically connects the second input terminal and one terminal of thesecond capacitor, electrically connects the other terminal of the firstcapacitor and the first output terminal, and electrically connects theother terminal of the second capacitor and the second output terminal ina first period, wherein the first chopping circuit electrically connectsthe other terminal of the first capacitor and the non-inverting inputterminal and electrically connects the other terminal of the secondcapacitor and the inverting input terminal in the first period, whereinthe second chopping circuit electrically connects the inverting outputterminal and the first output terminal and electrically connects thenon-inverting output terminal and the second output terminal in thefirst period, wherein the semiconductor device electrically connects theone terminal of the first capacitor and the first output terminal andelectrically connects the one terminal of the second capacitor and thesecond output terminal in a second period, wherein the first choppingcircuit electrically connects the other terminal of the first capacitorand the non-inverting input terminal and electrically connects the otherterminal of the second capacitor and the inverting input terminal in thesecond period, wherein the second chopping circuit electrically connectsthe inverting output terminal and the first output terminal andelectrically connects the non-inverting output terminal and the secondoutput terminal in the second period, wherein the semiconductor deviceelectrically connects the one terminal of the first capacitor and thefirst output terminal and electrically connects the one terminal of thesecond capacitor and the second output terminal in a third period,wherein the first chopping circuit electrically connects the otherterminal of the first capacitor and the inverting input terminal andelectrically connects the other terminal of the second capacitor and thenon-inverting input terminal in the third period, and wherein the secondchopping circuit electrically connects the non-inverting output terminaland the first output terminal and electrically connects the invertingoutput terminal and the second output terminal in the third period. 2.The semiconductor device according to claim 1, wherein the switch, thefirst chopping circuit, and the second chopping circuit each comprise atransistor, and wherein the transistor comprises a metal oxide in achannel formation region.
 3. An operation method of a semiconductordevice comprising: a switch; first and second capacitors; first andsecond chopping circuits; an amplifier; first and second inputterminals; and first and second output terminals, wherein the amplifiercomprises a non-inverting input terminal, an inverting input terminal,an inverting output terminal, and a non-inverting output terminal,wherein, in a first period, the semiconductor device electricallyconnects the first input terminal and one terminal of the firstcapacitor, electrically connects the second input terminal and oneterminal of the second capacitor, electrically connects the otherterminal of the first capacitor and the first output terminal, andelectrically connects the other terminal of the second capacitor and thesecond output terminal, wherein, in the first period, the first choppingcircuit electrically connects the other terminal of the first capacitorand the non-inverting input terminal and electrically connects the otherterminal of the second capacitor and the inverting input terminal,wherein, in the first period, the second chopping circuit electricallyconnects the inverting output terminal and the first output terminal andelectrically connects the non-inverting output terminal and the secondoutput terminal, wherein, in a second period, the semiconductor deviceelectrically connects the one terminal of the first capacitor and thefirst output terminal and electrically connects the one terminal of thesecond capacitor and the second output terminal, wherein, in the secondperiod, the first chopping circuit electrically connects the otherterminal of the first capacitor and the non-inverting input terminal andelectrically connects the other terminal of the second capacitor and theinverting input terminal, wherein, in the second period, the secondchopping circuit electrically connects the inverting output terminal andthe first output terminal and electrically connects the non-invertingoutput terminal and the second output terminal, wherein, in a thirdperiod, the semiconductor device electrically connects the one terminalof the first capacitor and the first output terminal and electricallyconnects the one terminal of the second capacitor and the second outputterminal, wherein, in the third period, the first chopping circuitelectrically connects the other terminal of the first capacitor and theinverting input terminal and electrically connects the other terminal ofthe second capacitor and the non-inverting input terminal, and wherein,in the third period, the second chopping circuit electrically connectsthe non-inverting output terminal and the first output terminal andelectrically connects the inverting output terminal and the secondoutput terminal.
 4. The operation method of a semiconductor deviceaccording to claim 3, wherein the switch, the first chopping circuit,and the second chopping circuit each comprise a transistor, and whereinthe transistor comprises a metal oxide in a channel formation region. 5.A semiconductor device comprising: first to sixth switches, first andsecond capacitors, first and second chopping circuits, an amplifier,first and second input terminals, and first and second output terminals,wherein the amplifier comprises a non-inverting input terminal, aninverting input terminal, an inverted output terminal, and anon-inverting output terminal, wherein the first chopping circuitcomprises first to fourth terminals, wherein the second chopping circuitcomprises fifth to eighth terminals, wherein the first input terminal iselectrically connected to one terminal of the first switch, wherein thesecond input terminal is electrically connected to one terminal of thesecond switch, wherein the other terminal of the first switch iselectrically connected to one terminal of the third switch and oneterminal of the first capacitor, wherein the other terminal of thesecond switch is electrically connected to one terminal of the fourthswitch and one terminal of the second capacitor, wherein the otherterminal of the first capacitor is electrically connected to oneterminal of the fifth switch and the first terminal, wherein the otherterminal of the second capacitor is electrically connected to oneterminal of the sixth switch and the second terminal, wherein the thirdterminal is electrically connected to the non-inverting input terminal,wherein the fourth terminal is electrically connected to the invertinginput terminal, wherein the inverting output terminal is electricallyconnected to the fifth terminal, wherein the non-inverting outputterminal is electrically connected to the sixth terminal, wherein theseventh terminal is electrically connected to the other terminal of thethird switch, the other terminal of the fifth switch, and the firstoutput terminal, wherein the eighth terminal is electrically connectedto the other terminal of the fourth switch, the other terminal of thesixth switch, and the second output terminal, wherein the first choppingcircuit has a function of bringing the first terminal and the thirdterminal into a conduction state and a function of bringing the secondterminal and the fourth terminal into a conduction state in a firstperiod, wherein the second chopping circuit has a function of bringingthe fifth terminal and the seventh terminal into a conduction state anda function of bringing the sixth terminal and the eighth terminal into aconduction state in the first period, wherein the first chopping circuithas a function of bringing the first terminal and the fourth terminalinto a conduction state and a function of bringing the second terminaland the third terminal into a conduction state in a second period, andwherein the second chopping circuit has a function of bringing the fifthterminal and the eighth terminal into a conduction state and a functionof bringing the sixth terminal and the seventh terminal into aconduction state in the second period.
 6. The semiconductor deviceaccording to claim 5, wherein the first to sixth switches, the firstchopping circuit, and the second chopping circuit each comprise atransistor, and wherein the transistor comprises a metal oxide in achannel formation region.